Travel from London to Stonehenge to Cornwall

One thing to consider is if you don’t drive manual, the car will be much more expensive to rent. Also, the gas in Europe costs about twice as much as the US. However, if you want to stop by Stonehenge, driving would obviously be the most convenient. If you decide to take the train, definitely book early as tickets get much more expensive the closer to the date. They usually have group discounts for buying 3 or more tickets at a time, so thats another way to save money if taking the train. I think it’s best to just calculate the cost of both and weigh them against each other taking into account conveniences and inconveniences of both options.

With that many people and luggage you would need a really good size car. I would not want to drive in London even in a smaller car and I think a car big enough for all of you would be difficult. Also as stated you would be driving on the left. I think there are also restrictions on driving in London so hopefully one of our English professionals can help you with that.

A 6 seat car is a relatively rare beast.
Are you traveling straight back to London after the 3 nights in Portleven ?
Car rental prices are lower the more days you have it.

I'm normally a huge proponent of public transportation, and the size of the travel party may make it difficult to rent a suitable vehicle, but this trip would be faster by car. The travel time between London and Salisbury is similar via train and car--if you exclude time needed to pick up the car, of course. But if you don't drive to Salisbury, you'll need to take the bus or a taxi to and from Stonehenge. You'd have to take time to stash your luggage in Salisbury, I assume, and then you'd have to retrieve it before continuing your trip.

Between Salisbury and St. Erth the train is apparently considerably slower than driving; it seems things slow down beyond Exeter. The question is: Can you be confident of finding a suitable vehicle in Salisbury or Exeter? Even if you had a strong preference for traveling as far by train as possible, what sort of vehicle selection would you find in Plymouth or Truro? I do not have answers to those questions.

Edited to add: It's clear from the comments of UK residents that the train might well be faster than driving, at least if you skipped the detour to Stonehenge. (It doesn't interest me at all, so I have no personal experience with going there.)

I suggest you hire and return the car at Heathrow airport. You should get a good range of car choices there and it’s easier to take the Tube out to Heathrow from central London to pick up the car than to drive in central London and out west. Heathrow is also right on the various roads down to Stonehenge and Cornwall. You’ll have to hire car seats for the toddler and baby too.
However, resign yourself to a long drive on busy roads - last time I drove back from Cornwall it took me 7+ hours. Just give yourselves enough time and allow for breaks for the kids!

You either must be brave/mad to attempt such a long tedious journey with children so young or they must be excellent travellers! Three nights only means 2 days there as it will take the best part of a day in each direction. Porthleven is a fair way west in Cornwall and driving from north London is likely to take between 6 and 8 hours with only a brief bathroom stop. I got unlucky once in traffic and Plymouth to London took me over 10 hours and this is a shorter journey. Don’t attempt it at the weekend in the summer.

Do you really have an interest in Stonehenge? It takes several hours to see it (it wouldn’t be on my top 100 places to visit list in England). But it’s not my trip!

As you have already considered, there is no easy option. For a party of your size, you are going to need either 2 cars or one large people carrier. The latter will be more difficult to source and you may have to go to Heathrow airport to find so you have the hassle of all getting out to Heathrow. I don’t know where you will be heading in Cornwall, but there are many narrow lanes there, so a smaller vehicle would be easier to manage.

Petrol/diesel is currently around £1.30 per litre (£5.85 per gallon) and seems to be increasing at a penny a litre every week as more people have started driving again after lockdown. It will be significantly more expensive on the motorway and in rural parts. Pre Covid, fuel was around £1.50 a litre.

Children under 4 travel free on trains. Buying seats 10 weeks out and travelling after 10am is the cheapest option. Hiring a large people carrier may not be possible in Exeter.

There isn’t one solution for you - it just depends on what your party wants/ is prepared to put up with. The journey is long whichever way you try to get there. I don’t have young children and I am U.K. based so know the roads and drive a manual and I would have to think twice about undertaking such a long journey for such a short time there.

you will have noticed that the M5 ends in Exeter - no motorways in Cornwall.

The A38 is the main road past Exeter and gets very busy, particularly on August weekends. Will the trip be midweek both ways?

The latter will be more difficult to source and you may have to go to
Heathrow airport to find so you have the hassle of all getting out to
Heathrow.

On the other hand, getting out to Heathrow saves you the hassle of driving out of London.

For both the baby and the toddler car seats will be required which both take up an entire seat space in the vehicle so you will need at least a six seater or two cars. One car would be preferable to avoid losing one another and also to enable a change of drivers however six seaters and above carry a higher premium and they're not common in smaller rental agencies. Depending on how you feel driving on Cornwall's narrow roads you might want to eschew a larger car.

Clearly none of us know the reasons for you wanting to visit Porthleven but for me, Cornwall is a place I would only consider driving to for no less than a week. It once took me six hours to drive from Plymouth to Portsmouth due to heavy traffic and roadworks, I had two moaning teenagers in the back, I'd hate to think what it would be like with a toddler and a baby. Don't make the mistake of looking at a map of the UK and thinking reasonably short distances in comparison to the US make for quick driving.

If you have a genuine interest in Neolithic history then Stonehenge is worth a visit. If it isn't then it's a big deviation from your route and will take up a lot of time notwithstanding it will be of no interest to the kids.

Those from the UK provided good responses. I agree with renting the car at Heathrow for a better selection. It’s easy to reach via the Piccadilly tube line. While they are more expensive, automatics are not that much more expensive. The piece of mind not having to shift with the left hand while driving on the left and navigating unfamiliar roads more than offsets the additional cost. The cost of fuel is what it is and you accept it when renting a car. Vehicles tend to get good gas mileage so you probably won’t fill up as often as you’d think and fuel costs are nominal when calculating the total cost of the trip. The easiest part of the drive would be Stonehenge/Salisbury and then down to Exeter. In Cornwall the roads seem narrower and there are more curves so it takes a lot longer to get to your destination e.g., 13 miles might take 45 minutes. Finally, many cars in Europe tend to have smaller trunks/boots than in the states so pack as light as possible using the smallest (carryon) size suitcases you can or everything may not fit. Leave those large suitcases at home.

I'd add Car & Driver to the option set. It's certainly worth a look, at least, seeing as the other two will already be cumbersome and not cheap.

A friend has used My Day Trip (https://mydaytrip.com/all-routes/gb) for a few travel day/transfers (France, Italy and somewhere else). Looks like they offer in United Kingdom. You might check for a customized plan.. Might be good for the day that you wish to stop in Stonehenge. Some I looked at for Portugal are basically a transfer from city to city and you could add a stop or 2 on the route.

Do all the adults want to go to Stonehenge? Perhaps 2 adults who really want to go could take the train to Salisbury. You can get timed tickets for the Cathedral and Stonehenge in Salisbury, including a bus ticket to the Henge. Since you can only view the Henge from afar; it really wouldn't be that exciting to the less interested; especially small children. Then you could rendevous at that night's accommodation. I'm not sure if there is a train station anywhere near Porthleven as they just have a bus listed. So they might have to rent a car (perhaps in Salisbury) or take a taxi. Or 2 smaller cars from the start (not in London) could be the solution; with tradeoffs with the kids. Also giving you opportunities for later nights out, taking turns with baby sitting. Small cars with trunks, so you can keep some stuff there while touring. Good Luck!

Thank you to all for the wonderful information! It sounds like the verdict would be car over train, and 2 cars at that, which we are completely good with. As someone mentioned there is a chance we are a bit mad to be attempting the journey with such little ones, though they are relatively good travelers. :) Google maps was saying it would take 5 hrs which didn’t seem like that bad but it seems like that can be pretty uncertain. We are heading down on a Monday so we’ll have rush hour traffic getting out of London but hopefully once we get to Heathrow to pick up the cars (thank you all for that suggestion!) traffic won’t be as bad. I don’t have a huge desire to see Stonehenge, but others in the group did and it seemed to be right on the way (again according to google maps). If this will be a huge inconvenience perhaps I can talk them out of it. I am also now considering flying from Heathrow to Newquay to avoid the drive all together. After our 3 nights in Porthleven we are staying 2 nights near St Anne’s Chapel and Tamar Valley to explore a bit more of northern Cornwall. I know these “home bases” may not be completely ideal but we were limited on lodging to accommodate all of us with amenities for the babies. From there we will be driving to the cotswolds and spending the last 3 nights there.
Thank you again for any and all suggestions!!

If you did drive the worst part would be the M25 parking lot and you’d only be on it for a very short distance before picking up the M3. At that point you’re heading away from London and traffic going into it. Hopefully there’s little construction on the M3 and/or the A303 since you could lose a lot of time sitting in construction traffic and traveling at a low rate of speed. From Heathrow to Salisbury it’s only 64 miles on good roads, so shouldn’t take longer than 1.5 hours, and should be an easy drive.

So I think we have all been convinced to nix Stonhenge from our itinerary and just make the trip from London to Porthleven. With Stonhenge removed does that make the train (possibly from Paddington to Penzance) a better option than driving? It looks like they should both be about 5hrs but I like that is a more guaranteed 5hrs on the train.
Edited to add: we would still rent a car in Penzance for the remainder of our trip

Think with the extra days and including the Cotwolds I would be tipping towards car rather than train.
That second rental being so close by is very convenient swings the "covering the miles" time to fun time well into your favour.

Google maps say Heathrow to Penzance by car is 4.50, that probably means 5.30-6 hours in reality. To that you need to add the time get to Heathrow (or some other car rental location) and collect the car. The train seems like the best option if you don't want to stop at Stonehenge.

If its me and Stonehenge was now out of the equation, I'd take the train. Its just much more relaxed, IMO. And I would rather know when I am due to get in vice dealing with traffic, road works (construction), narrow roads, parking, etc. You can also get up and move around during your journey.

what Tom said

and .. on the train the kiddoes can move about a bit rather than being strapped in.

So much easier by train than a long drive for them. There are even changing tables in the universal toilet...

where in the Cotswolds?

Consider taking the train down to Exeter, avoid all the terrible traffic. Rent you car at Exeter, spend your 3 nights there, then drive to Salisbury, Stonehenge and return the car at Heathrow.

London to Penzance on the Night Riviera!

One of only two sleeper trains in Britain!

Lemme put in a plug for Stonehenge and Salisbury Cathedral. Salisbury is a wonderful town and the Cathedral is amazing and very friendly and welcoming. It has a copy of the Magna Carta! Stonehenge is gorgeous and evocative. Salisbury has the Old Sarum ancient site. Salisbury is a nice short train ride from London and has a convenient care hire near the train station. There are beautiful huge chalk horse figures carved into the hillsides around Wiltshire, which you will see as you drive around.. Such a lovely area. Easy to get to the Cotswolds from there.

I recently listened to 'Sarum', by Edward Rutherfurd, and now I can't wait to go back and spend more time there!