train or bus Cardiff to London

The best place to search for UK train tickets is http://nationalrail.co.uk. When you go to purchase, they'll send you to the site of the actual provider, which in this case, yes, is Great Western Railway.

Where are you finding a bus that takes only 25 minutes longer than the train? Both National Express and Megabus list the travel times from Cardiff to London at 3 1/2 hours or more (depending on time of day and therefore traffic), whereas the train takes just over 2 hours. But the bus is indeed a lot cheaper, so if cost is a factor for you, it's not a bad choice. I've traveled a fair amount on UK intercity buses (or "coaches"), and for short trips, they're absolutely fine. The schedules are padded to account for traffic, so you'll arrive at your destination early more often than not.

Thanks for the info! The time for the bus popped up on the screen in the right hand corner when I was on the trainline website. I will definitely look into it as an option.

This is the site you need for your train tickets:>https://www.gwr.com

Best to pre-book specific trains about 11 to 12 weeks ahead for lowest prices. If you are paying on or nearer the day, you may find it cheaper to but split tickets with the split being at Swindon. (You stay on the same train - just click details on www.nationalrail.co.uk and it will tell you the stopping points - all trains stop at Swindon but you may also find that yet another split - at Didcot will reduce the fare even more if paying late. Not all trains stop at Didcot - it must be a stop to split the ticket even though you say on the same train).

Generally, the trains run precisely to the minute at speeds up to 125mph. Much work is going on at the moment to electrify the Great Western Mainline from Paddington to south Wales. You therefore may encounter engineering works on weekends - especially Sundays. If the train is not going to take about 2 hours, you are going on a diversion.

Megabus & National Express can encounter delays on the M4 motorway (like an interstate) - especially at the Newport bottleneck where 3 lanes each way become 2 to get through a tunnel.

"Megabus & National Express can encounter delays on the M4 motorway (like an interstate) - especially at the Newport bottleneck where 3 lanes each way become 2 to get through a tunnel."

Again: The schedules are designed to account for normal, predictable traffic like this. It's not the case that the 3 1/2 hour travel time is the most optimistic possible scenario and every little delay adds onto it, so the typical time is more like 4 or 4 1/2 hours. Rather, the optimistic scenario is maybe 3 hours, so they give it 3 1/2 in the schedule, and there's a good chance you'll get there in 3 1/4.

Is it possible that traffic will be worse than they anticipated and you'll be late? Of course it is. But that's a rare event. It's also possible (and about as unlikely) that a train you're on will break down.