I think Cirencester is the right choice for you given what you want to see and where you want to go.

Kemble is a small station in a country village. However there is almost always at least one taxi waiting to meet the London trains. If by some remote chance there isn't, then either ask at the ticket office (staffed until 13.30pm) or there will be information on the notice boards.

http://www.nationalrail.co.uk/stations/kem/details.html


But you say earlier you will be leaving London early on the Thursday morning so you really shouldn't have a problem.

Otherwise you could always google Cirencester taxis and pre-arrange a pick-up. Here's a couple to get you started...

http://www.cirencestertaxi.com/cirencesterwhere2.htm (charge £11 for station pick-up)

http://a2btaxisofcirencester.com/

http://www.cirencestertaxis.co.uk/

You will need to retain a taxi number to call when you drop off the rental car on the Sunday to take you back to Kemble Station.