Need help getting out of London to rent a car.

Depends how far "out" you want to go (and are prepared to pay for train tickets to get to).
The obvious choice is Heathrow, it is in the right direction and on the edge of London. It is just inside the M25 (London Ring Motorway), but the Heathrow area can get busy.
Otherwise I would pick somewhere where you want to spend a day. You could get the train there, explore the place, stay overnight and pick up the car the following morning. Suitable places are: Bath, Salisbury, Winchester, ...

Option 3 is train all the way to Cornwall, and pick up the car there.

If you want to pick up a car in Penzance, for instance, then work your way back along the coast (or vice versa), you need not return the car to the same point; see www.kemwel.com for a good selection of rental locations. If you want to pre-book a long train ride with advance purchase discount, then visit www.nationalrail.co.uk about 2 months in advance of the train travel date. There are a few direct trains per day between London and Penzance and others with a connection at Exeter or Plymouth. Closer destinations have more frequent (and cheaper) train service.

My wife and I flew into London, took the train to Bath immediately, visited Bath for 2 days, and then took a car rental from an Enterprise location in Bath and drover to Cornwall (easy drive - mostly on multi-lane interstate type roads). You're right, in my opinion, not to rent the car in London. Bath is about a 90-120 minute train ride from London, worth a visit itself, and makes the drive to Cornwall (e.g., St. Ives) about 4 hours. It'd be a long day, but you could do London to Bath early morning, visit Bath until later in the day, and then drive to Cornwall, particularly if you visit in summer when the daylight hours are long and make driving easier. If you don't want to visit Bath, consider getting outside London just a bit to a smaller city west of London that is well connected by train(e.g., Reading) and picking up a car there. Otherwise, I can vouch for the sleeper train between Penzance and London - take the train to Penzance, rent car in that smaller town, and then drive to your Cornwall hotel/destination. You will love Cornwall! We certainly did. Cheers and happy travels.

There are a few practical considerations to bear in mind --- Do you drive a stick ? When renting a car you will have to specify an automatic transmission if you want one , and some smaller locations may not be able to guarantee that you will get it . Even if you do drive manual , keep in mind that you will be shifting with your left hand , Some renters find this confusing , others , not . At Heathrow , there is a very good likelihood that you can count on an automatic , which is then one less thing to be thinking about as you become accustomed to the roads in the UK . It's not impossible , but there is a learning curve to it ,and some people adapt easily and others , well , not so easily . This site , kindly provided by MC on the Scotland thread is good information ! https://www.gov.uk/browse/driving/highway-code , ------- https://www.learnerdriving.com/learn-to-drive/highway-code/road-signs

Agree with Steven about the automatic -- the one time I rented in Britain I drove a stick VW at home so I was very used to shifting with my RIGHT hand but rented an automatic at Heathrow to reduce the stress of combining left-handed shifting with left-sided driving.

Thank you everyone for the great information. I have a few more questions. I'm considering the train from London to Bath. Where in London do we get the train? How close to the train station in Bath is the car rental place? I'm assuming it won't be too hectic driving out of Bath.

We both can drive a stick so we should be ok if we don't get an automatic.

Can anyone recommend places to say in the St Ives area?

Thanks again.

pmack,
Trains to Bath leave from London Paddington station (where the bear comes from :-) )
If you are staying overnight, the question is not how far the car hire is from the station, but how far it is from your hotel. And that can only ne answered once you decide which hotel and which car hire company to use.
Bath is a medium sized town (population 88,000), driving out of it should not be a problem, so long as you know where you are heading for.

PMack, the train station is in central Bath, near the main attractions (Abbey, Roman Baths, etc.). The Enterprise location we rented from was closer to our hotel, but probably 45 minutes walk from downtown (on Lower Bristol Road). Good thing is that is a very quiet area and a great place to get your sea legs under you if driving there for the first time. As for places in St. Ives, we stayed at Primrose Valley Hotel which was nice (small rooms, but nice). There's a really steep driveway off the main road to get to the parking lot next to the hotel, but other than that it is well situated very near the beach and the town of St. Ives. Note that St. Ives is very touristy the closer you are to the harbor front, but even a block or two off the harbor front the twisting lanes, shops, pubs, etc. become much more charming.