London to Brockhampton Xmas

The important question here given transport shutdowns etc over Crhsitmas, is what are your exact travel dates?

In case you didn't know, there will be no trains on Xmas day and limited trains, if any, on Boxing Day.

Compare costs of picking up a car and dropping in at the same location vs dropping it at a different location. There is usally an extra charge at the latter.

Also check the times the rental offices are open and you can pick up and drop off cars.

But even on Christmas Day National Express run buses to Cheltenham (and around the UK) from London Victoria and Heathrow Airport.

On Christmas Day just 2 buses, but normally an almost hourly direct service from London and Heathrow.

Between Christmas & New Year there are no trains running through from Cheltenham to London, only a shuttle going as far as Swindon. You could change there for a service to London Euston but this would take an hour or more longer than usual. Not attractive.

From Oxford in that period there will be a regular service to Marylebone.

To be clear there will still be trains to Reading from Swindon (as well as the Euston trains, with no passenger stops between Swindon and Euston).

Trains are then running from Reading to Slough, for a non stop rail replacement bus to Hillingdon Underground Station (Met and Picc line services to Central London) or an all stations bus to Ealing Broadway for District and Central line tube connections to central London. The Ealing all stations bus will be slow (75 minutes), but the Hillingdon bus only takes 25 minutes.

Plus the regular Reading to Waterloo trains and being allowed to go on to London via Guildford & Basingstoke.

Lots of alternatives have been put in place but they all have some, or a lot of extra time & inconvenience.

Thank you for all your tips. Fortunately we are leaving London on the 23rd and returning on the 27th. The idea is to avoid doing the drive from London and rent a car somewhere along the way to Brockhampton (we have been thinking Cheltenham Spa - closest train station to Brockhampton or Oxford - just along the way)

Option 1: Cheltenham Spa: Plan works for trip out on the 23rd, but no trains from Cheltenham Spa to London on the 27th so options for return to London on 27th are 1) bus 2) drop rental car at a different city and take train to London from there

Option 2: Oxford: Trains available from 23rd / to 27th London and car rentals are available near train station. Just have to check if the car rental places are open those days.

Any opinion if taking train London from/to Oxford is better on 1) London Paddington (via Great Western Railway) or 2) London Marylebone (via Chiltern Railways) ?

Given that there are no Paddington trains to Oxford on the 27th due to the work at Old Oak Common (trains to Reading only) then it is Marylebone on the way back, or the same situation as for Cheltenham - extended journey times with part by bus or train from Reading into Waterloo.
The Chiltern route to Marylebone will have longer trains than normal, but still be under a lot of pressure.
If it was me I would use the Oxford Tube express bus service from Oxford to Victoria, which will also be busier than normal.
To add insult to injury the diverted trains to Euston are calling at Oxford, but for staff purposes only, not picking up passengers.