Renting a car for one day outside London

Assuming you mean Highclere, out south of Bloomsbury:

. It's an hour and a half each way.
. You need X hours there.
. You want to see other things totaling Y hours.

That's almost your whole working day already. You don't have time to migrate past traffic and get a car. Pick one up close to where you're staying and choke the congestion charge.

Hello

I assume that you mean Highclere Castle, the site where Downton Abbey TV series has been filmed? If so, it is about an hour's drive west of Heathrow Airport, just south of a town called Newbury, in Berkshire. Thus you could hire from the airport (which is on the tube) and drive from there. However if you are staying any further into London, self-driving probably does not make much sense. If you do decide to hire from Heathrow or nearby, you could also pass Windsor Castle or, perhaps a more interesting place off the beaten tourist track (and quite close to Highclere), Stratfield Saye House and Estate.

Yes, I meant Highclere Castle. We will already be staying in London near Hyde park and then have the tickets for the castle. I wondered if we should get the car on the outer part of the city as it would be an easier drive. I have driven in Dublin and survived o.k.

Thank you, Kathy

Definitely take the tube out to Heathrow and rent the car there. Driving in London is a huge waste of time, very stressful and completely unnecessary.
Getting out to Highclere, seeing the castle, walking the gardens, and having tea or lunch, will be an all-day affair!
Make sure to factor in the time it will take to take the tube to Heathrow (1 hour; make sure you know which terminal to get off at for your rental car), find and procure the rental car (30 mins at least), drive out to Highclere (90 mins at least), see the castle and grounds (3 hours minimum), drive back to Heathrow (90 mins again), return the rental car (30 mins), and then take the tube back to the Hyde Park area (1 hour). Very full day.
When we went to Highclere we had tickets for the afternoon so we got a very early start and spent an hour or so at Stonehenge on our way - we were heading to Oxford for the evening though so didn't have to factor in getting back to London. A Stonehenge visit will take a little longer now I believe, since they have installed a visitor center about a mile away from the stones and you have to walk there or take a shuttle bus with timed tickets.

What time is your reservation at Highclere Castle? What day of the week?

Why don't you simply go to Paddington main line station & get on a train to Newbury and then take a taxi to Highclere Castle? The trains only take 52minutes to just over an hour & the taxi ride is only about 15 minutes. Travelling Monday > Friday, the cheaper £23.10 fare kicks with the 9.36 departure (or later). On this particular train, you must change at Reading - so make a note of the platform numbers (though they can sometimes change). www.nationalrail.co.uk
http://www.highclerecastle.co.uk/visiting-us/planning-your-visit.html

"What time is your reservation at Highclere Castle? What day of the week?"

Exactly, it's often mentioned that it's fully booked for the whole of 2014, so I hope you've pre-booked tickets but there is talk of very limited "walk up" tickets on the day.
BTW it's the Cotswolds.

My purpose for those questions is that it may affect driving time and costs.

My only comment is that it's quite easy to drive a rental car out of Heathrow - the signage is quite good and if you have a GPS it's even easier.

Thank you for all the suggestions . We have afternoon tickets on a Thursday. I purchased the tickets on line in feb.

Thanks to all.

If mid afternoon it should not be too much trouble. The roads and motorways out of London start to clog up around 3:00 to 3:30 (school run) and then get much worse with commuters around 4.

We didn't have time, on a recent trip to London, to consider the "Call the Midwife" location visit, also well out of town. Again, it can be done by public transportation.