Last day help! :)

"Spend the day in London. We have been to London twice so we have explored a lot of the popular spots, BUT we have never seen the Christmas displays or been to Borough Market. If we do this, we will return the car early and just use public transportation to get around! "

This is the way :) I love 1 and 2 but neither seems that close with heading back to London from the Cotswolds. You could do Oxford/Blenheim, but London at Christmastime is magical.

A few more details might help get you good advice. What time of year? You mention Christmas so November/December? From where in the Cotswolds are you departing? Where are you staying the last night before your flight? What time is your flight out? Where do you need to return your rental car?

Not knowing the answers to these questions, given your description I myself would probably skip London, book a hotel near the airport for your last night and drop off the car. I'd select a stop or two that is on the way to the airport from where you're leaving. Some of those options are pretty far off the path for a day's drive, especially in a low-daylight-hour time of year with potentially iffy weather. But that's just me.

It’ll take a long time to drive to the Jurassic coast and then back to London, Canterbury would (probably) be a bit quicker but neither is practical to my mind. Oxford or Blenheim (but not both) would be quite doable. You mention Xmas displays- bear in mind that it’ll be dark by around 4pm.

Personally, I’d go for Oxford. But, a car is a liability in Oxford so park in one of the “Park and Ride “ car parks on the edge of town and get the bus in.

A few more details might help get you good advice. What time of year? You mention Christmas so November/December? From where in the Cotswolds are you departing? Where are you staying the last night before your flight? What time is your flight out? Where do you need to return your rental car?

We will be in London the last week of November. We will be driving from Bourton on the Water and again can leave as early as needed. We are staying at a hotel near LHR and our flight leaves at 10am. We will be returning the car that am or the day before depending upon on final plan. If we spend the day in London we will just return the car early and take public transport around the city and to the airport in the AM so we don't have to worry about it!

I would say Oxford. Enjoyment of the Jurassic coast is going to be very weather dependent, and you don’t have many hours of daylight at that time of year (sunset around 4pm) so even on a nice day you want to be tucked up in a cosy pub by early evening.

It seems a shame to not see London in its holiday attire. Are you able to leave your luggage at the hotel near the airport, drop of the car as early as possible, and then enjoy the day (and early evening lights) of Christmastime London? And be safely near the airport for the 10AM flight?

You could just leave it unplanned until you get to it. On one of our recent trips to the UK, we left the last 3 days completely unplanned. We decided what we wanted to do when we got to those days and they were some of the best days we had.

Other Idea: Christmas at Kew this is an amazing experience with lights.

https://www.kew.org/kew-gardens/whats-on/christmas

Canterbury is on the opposite side of London from the Cotswolds, suggest forgetting that idea.

Oxford or Blenheim Palace would work.

Because of the early flight time, I recommend booking a hotel that is in your departure terminal, not just "near LHR." Drop the car, drop the bags and head into the city for the day. This is the type of situation in which I would be looking at a late afternoon/evening ride one of the buses (HOHO, the tea bus, Bustronome, or just the city bus) and/or the Thames boats as a way to see the holiday lights.

I agree with G3rryCee. If you can change your "near LHR" hotel to one in your terminal, this is the best option. Staying in your terminal or directly next to your terminal is a gamechanger---so much less stress than even one of the many just "down the street".

last week of November. ... at a hotel near LHR and our flight leaves at 10am ...

If you try to return your car in the morning you will be doing it in the dark - not best for the return camera shots and walkaround.

Sunrise this week is about 8am where I am and even with the clocks changing, a month from now it won't be any earlier and you need to be in the flow of things in the airport at 7am for a 10am flight