Renting car at Heathrow traveling to Bath

All I can say is that we always rent at Heathrow when we are going into the countryside - return the car there. It's quite easy to get in and out of the airport.

I wouldn't recommend returning the car in London; combination of lots of traffic, having to pay a Central London congestion charge (£10), difficulty finding the rental car location, and possibly an extra fee (£40ish) for dropping the car off at a different location.
If you're not locked in to having a car, think about public transportation. A National Express bus from Heathrow to Bath is about £15 for an adult and takes about 2.5 hrs. A train from Bath to London (Paddington station) takes about 1.5 hrs.

We've rented a car at Heathrow and have driven to Bath. We were there for five days as my wife was participating in the Jane Austen celebration and Regency promonade through the city. We had wonderful day trips in the car to Stourhead Gardens, Castle Coombe and other locations. Parking is not cheap but you can park overnight at a few large lots. Here's a LINK to a map. There is a nice lot next to the Police station. Parking is about 20pounds/day and you can park overnight. You just keep feeding the meter and placing a ticket on your dashboard. Our drive went throughout England and we did return our car to Heathrow. Very easy. Should you return your car to Heathrow, you can take the underground Piccadilly line all the way to the Kings Cross train station. St. Pancras is literally next door. You will have your luggage with you and their are nice escalators at Kings Cross. Or, if you rent from either Europcar or Autoeurope, they have a location at the Kings Cross station. I haven't use any of them there but they do list locations there. Hertz does not appear to be there. I don't see Avis there either.

Thanks for the information, I wasn't sure about driving to St. Pancras so that is helpful. Any idea how long it will take to drop off the car at Heathrow and get to the train station? We have tickets already for Paris. We are planning to go to Avesbury or Stonehenge, any thoughts?
Thanks

If you rent the car at Heathrow and drive to Bath, Salisbury (Winchester Cathedral), Stonehenge and Avesbury are kind of on the way. We hit them all of the way to Bath. Hopefully, you can turn your car in at Kings Cross. Note that you have to be ready for check-in for the Eurail train to Paris 30 minutes before departure so you really need to be there a good 45-60 minutes early. If you turn in your car at Heathrow, I would estimate that the entire process of turning in the car, walking to the underground, taking the Picadilly line to Kings Cross and exiting Kings Cross for St. Pancras to be a good 2.0-2.5hrs.

OOPs Larry, Winchester cathedral is in Winchester, Salisbury cathedral is in Salisbury. Still i agree with larry, fairly easy to do Salisbury cathedral, Stonehenge and Avebury on the way to Bath. Dont try and do it all on the day you leave heathrow, all these sights are worth spending time in. PS: Both Salisbury and Winchester cathedrals and towns are worth visiting.

Thanks for the great info. I think it might be easier then to drop the car back at Heathrow and take the train rather then fight traffic to get to St. Pancras.

The western route into central London is busy (traffic to and from Heathrow uses it) and is often subject to delays, more so in the commuting hours. So IMHO your decision to return the car to Heathrow is wise.