I checked the route from LHR to Swindon.
1.) Express Train to Paddington
move to
2.) GWR Paddington to Swindon
No GWR timetable for summer, yet.
I overthink to travel first class by GWR. What do you think, how expensive it will be? What happened if the flight will be delayed and I have to travel later by GWR? Is it allowed to use a later train with the tickets I bought? Or have I to buy new tickets?
check the website The Man in Seat 61 for how British train tickets work, and possible discounts. Summer is a long, long ways away but timetables don't change all that much on major routes.
There is no need to take the Heathrow Express from Heathrow to Paddington, although if you book well in advance you can get a decent deal. You can also take the Elizabeth line.
But you could take the Elizabeth line one stop and change for a train to Reading and then to Swindon.
Or take a coach from Heathrow to Reading for a train to Swindon. All three routes will work.
You can get cheap fares on the trains but if you buy an Advance fare and miss the train you will have to buy a new ticket at the walk up price. You can buy a walk-up ticket tomorrow from Paddington for £35.10 (prices will rise in March).
There is absolutely no need to travel First Class on this journey. It takes less than an hour from Paddington to Swindon and obviously less from Reading.
The cheaper advance fares tie you to a particular train and after arriving at Heathrow, I think you will find it difficult to fix your time.
Are you going to return to Heathrow within a month? If so, it is likely to be cheaper for you to buy a return. This can be had for £58.70 if you take the Elizabeth Line from Heathrow to Hayes & Harlington and then a stopping train west to Reading where you change for a fast train to Swindon. That fare applies to the off peak times in the middle of the day (weekdays) but any train on weekends.https://www.gwr.com
The easiest option of all is National Express bus from Heathrow direct to Swindon in 90 minutes, every 2 hours or so. No changing anywhere, let alone twice, as the Elizabeth Line option requires.
The benefit of the Heathrow Express is that it is often the cheapest option if booked ahead, due to the Airport premium on Elizabeth Line fares.
