Update ...
Let's see if I have this right ... as I'll be sharing on our Facebook page once my classic spelling errors have been corrected.
GWR▸ train services over Christmas 2018 and the New Year 2018/19
4 major set of engineering works on dates shown in tables.
Overview of engineering changes on red dates:
WESTBURY 23rd December 2018 to 3rd January 2019 inclusive
Portsmouth to Cardiff trains replaced by buses between Trowbridge and Warminster
Swindon to Westbury trains replaced by buses between Chippenham and Westbury
London to West of England trains that call at Westbury will call at Frome instead
Bristol to Weymouth trains will not call at Westbury
South Western Railway trains from Bristol to London (Waterloo) via Salisbury will not run
PADDINGTON 23rd December to 26th December inclusive also 30th December
Long distance trains from the South West, Bristol and South Wales will terminate at Reading
More local trains will only run as far in as Slough - no trains from Slough to Paddington
An exhanced train service will run from Reading to Waterloo
Passengers from the North Cotswold line map prefer to change at Oxford and take a train in to Marylebone
Passengers from Exeter and west thereof may prefer to take the
SWR» train from Exeter to Waterloo
A wide range of rail replacement buses and ticket acceptance will in place between London and Slough and Heathrow
SEVERN TUNNEL 25th December 2018 to 1st January 2019 inclusive
GWR trains that normally go to Newport will terminate at Bristol Parkway.
(so that's Portsmouth - Cardiff, Taunton - Cardiff and London [Reading!] to Cardiff and Swansea
Buses from Bristol Parkway into South Wales
GUILDFORD 25th December 2018 to 30th December 2018 inclusive
Services from Reading that normally run via Guildford will terminate at Ash
Buses from North Camp (do not stay on the train to Ash) all stations to Redhill
Buses from North Camp (do not stay on the train to Ash) direct to Gatwick Airport
Most
South Western Railway services that call at Guildford all also be replaced by buses
London to Guildford via Effingham Junction trains
will run.
Note that Buses will NOT run on 25th or 26th December
General Timetable - special days
24th December - services will end early evening
25th December - no services
26th December - no GWR services. Limited Chiltern service Oxford to London (Marylebone)
27th and 28th December - some peak services will not run
31st December - some peak services will not run. Check online for evening
1st January - check online for morning services
There may be other engineering works too - we suggest you chcek all jourmeys online; timetables expected mid-November
Edit - minor spelling correction and clarification