Heathrow to Cornwell to Southampton

Why not get a train to Cornwall and hire a car there?

If you go by train to Penzance, in Cornwall, then Europcar have a depot literally at the station. So on arrival day at LHR you could train to Penzance, stay there overnight (lots of places to stay, but those include a Premier Inn opposite the station for that one night), then pick the car up next morning when you're fresh.
The Rosemundy House Hotel at St Agnes is a very characterful and historical place to stay. It's on the north coast, right in the heart of old tin mining country. You can't miss the ruins of the old mines. You can book direct with them on line.
Loads of beaches and good walks and a good central location for exploring the county.
Only a mile or so off the A30, the main arterial road in Cornwall.
Europcar have an office at Southampton Airport and it's a very short distance from the airport to the port. There are 5 widely spaced cruise terminals so when your tickets come you need to be able to tell the taxi driver which terminal.
It's 220 miles from Penzance to Southampton- route planner says 4 hr 28 min. In July allow at least 6.
So you need to go to SA the day before the cruise. There are lots of hotels very close to the three principal cruise terminals. But it would be far too risky to go up that morning.
So really you have three days in Cornwall.

isn31c:
That is incredibly helpful. We were planning on staying in Penzance, so renting the car there makes a lot of sense. We were thinking of staying in port Issac, but do you think that would be too congested with tourists (thinking doc Martin).

The rest of our cruise travel group is staying in Southhampton the night before, after going to Paris, so we may do that based on your recommendation.

Thanks very much for your suggestions!

A tip for Cornwall: In the popular coastal towns, at least, you will need to have dinner reservations unless you plan to get that meal from a fish-and-chips food truck--in which case, expect to have to fend off the seagulls. Lunch may be easier without a reservation, but I'm not sure about that since I never tried to find a sit-down lunch.

Port Isaac is more of a day tripper's village. So if you're out touring during the day you should miss a lot of the busyness.

Really anywhere in Cornwall is going to be busy, busy at that time of year. It depends whether you are there before the English schools start their Summer Vacation in the middle of the month.

PI is also known for the movie Fisherman's Friends- worth seeing both the first movie (2019) and the sequel which came out last year.

Bear in mind that parking in Port Isaac is extremely limited and with many places to stay you will need to park at the top and walk down what is a very steep hill. Choose carefully.

We will be doing some similar things in Cornwall, although in May, and just based on my digging, while hopping a train directly to Penzance sounds ideal; the fastest route looks to be about 6 hours, more than I want to sit on a train after a trans-Atlantic flight.

Depending on your itinerary. maybe a first nights stop closer might be worth looking at. I can handle a couple hours travel, even if a bit out of the way; Windsor? Bath?, even Bristol?; all sound like possibilities for me.

Yes, that 6 hour train ride, including a change in Paddington with all our cruise luggage is a bit much after the long flight. I’m not yet settled on that itinerary myself.

I do not think it really saves any time, still about six hours, but you can also take a bus to Reading, and catch the train to Penzance there, but it does save having to head into London.

Time wise the Railair coach via Reading is quicker from T5 and a wash from T2/3. If going that way buy the coach tickets directly.

That is very good advice about booking meals in advance. And by advance, I mean a few weeks or even months in advance. Cornwall is very busy in the summer and most restaurants have trouble attracting enough staff.

Until Flybe collapsed you could have connected into a late afternoon flight from LHR to Newquay airport arriving at 6 pm or 8.10 pm then stayed in Newquay.

But now you would have to transfer to Gatwick for a different carrier, (Eastern Airways).