staying at Cheltenham then to Blenheim and London

Certainly doable, depends on where you are dropping off the car. If at Heathrow, not a problem. Here is your route from Birdlip sounds like it uses the pub in the village as the starting point (lifted from the Blenheim Palace website) 61.8 km - about 1 hour 5 mins 1. Head south 0.1 km 2. Turn right toward B4070 0.1 km 3. Take the 1st right onto B4070 0.7 km 4. Turn left onto A417 1.4 km 5. At the roundabout, take the 2nd exit onto A436 Go through 2 roundabouts 10.5 km 6. Turn right onto A40 Go through 2 roundabouts 28.3 km 7. At the roundabout, take the 1st exit onto Burford Rd/B4047 6.0 km 8. At the roundabout, take the 1st exit onto Burford Rd/A4095 Continue to follow A4095 1.3 km 9. At the roundabout, take the 1st exit onto Bridge St/A4095 Continue to follow A4095 Go through 2 roundabouts 1.9 km 10. Turn left to stay on A4095 Go through 1 roundabout 10.3 km 11. At the roundabout, take the 1st exit onto Oxford Rd/A44
Destination will be on the left 1.3 km Oxford to London is quite easy, look for signs for the M4 - once you are on that it will take you straight to Heathrow or into London. Probably about 1 hour's drive depending on traffic. The Blenheim Palace website will give you directions on how to get to the M4. I hope you are spending time in Gloucestershire. The Cotswalds are lovely and full of interesting things to see - Roman villas, Bronze Age earth works, standing stones etc. And many, many pubs and restaurants.

If you haven't been there yet you could go to Blenheim via Stow on the Wold and Chipping Camden. Blenheim only takes an hour or so to tour. So just allow enough time to get to London from there. Is the time when the car is due back? Maybe you can turn it in at Heathrow and take train into town.

Bear in mind the problems using the M4/A4 because of cracks recently found in the supporting structures of the M4 approaching London causing the closure of the roadway between J1 and J2 and previously J2 and J3. It is causing massive tailbacks. The cracks are in very critical areas. There have been assurances that something will be done so that this doesn't cause logistical nightmares during the sporting fixture we will be having at the end of the month but nothing is certain - and it is not known what work will take place after the Olympics are finished. Just be aware... The airport is further out but the traffic because the motorway is closed has been reported as horrendous. Cheltenham is a great place to be from - I am. See if you can have a walk up on Cleve Hill...

Nice to see someone else who appreciates Cleeve Hill, I have climbed it a few times from Woodmancote ( G parents lived in Bishops Cleeve ) and still visit it to this day.

Thanks everyone. I'm adding all of this info to my guidebook! Have booked the car for one extra day. So we won't have to be in a hurry.

have you considered dropping your rental car off at Oxford or heathrow. Much less stressful, and the train from there to London is quick and efficient. The closer you get to London, the more horrendous the traffic is going to be