London Paddington station will be closed to all traffic due to engineering work on 24 to 27 December.
No trains would have run anyway on 25 and 26 December.
On 24 and 27 December main line trains will start at Reading.
For Oxford use Chiltern Railways from Marylebone.
For all other destinations there are two routes from London to Reading-
a. South Western trains Waterloo to Reading;
b. Tube to Ealing Broadway, then Elizabeth Line Ealing Broadway to Reading.
For Devon and Cornwall there is also the South Western trains route from Waterloo to Exeter St Davids (change for the rest of Devon and for Cornwall).