Full details of all rail workings over Christmas are on https://www.gwr.com/travel-updates/christmas-2016, but the key fact to note is that Paddington will be closed from December 24th to Thursday 29 December. This means that GWR rail services starting and finishing at Ealing Broadway, where travellers can connect to the underground, or bus services into London.
Ealing Broadway does not have step-free access. If travelling from the West Country therefore it may be better to change at Reading and use services into Waterloo.
Further details can be obtained from mark.hopwoodGWR-MD@GWR.com or phone 01793 499424