A lot of engineering work is coming up over the next few months on routes operated by GWR (& other companies). Most notably, GWR long distance trains will divert into London Euston (instead of Paddington) on certain dates. This is mainly because not far west of Paddington, a large new station is being built for HS2 = the high speed railway that is being built to connect London with Birmingham & hopefully beyond.
Click the following link to see if this will affect your proposed journeys:>https://www.gwr.com/travel-information/travel-updates/planned-engineering