Best Ticket Type for London to Cardiff?

I am planning on taking the train from London to Cardiff in mid-June, and then returning by train to London from Cardiff. The main BritRail site offers point-to-point tickets and 4-day Flexipass options, both of which are pretty expensive. The Great Western train offers tickets called Firstminute Fares, which seem like you purchase them in advance for a specific train time, and if you have to change the time, you pay some penalties. Any opinions on which offers the best deal for a traveller?

It really depends on your plans. The advance tickets are the cheapest if you know when you're going to travel, but aren't so good if you want to keep some flexibility. I think you can change the advance tickets for a fee (拢5?) though. Bear in mind that you can book advance ones up until 6pm on the day before you travel, though the cheapest ones won't usually still be available at that stage.

One tip is that you can often get cheaper tickets by booking single tickets and by booking the journey in stages rather than complete, eg I think Cardiff to Swindon only costs about 拢5. It's definitely worth doing a bit of research on a website such as http://www.thetrainline.com before making any definite decisions.

There's been some things in the press here about getting cheaper tickets and found the following article on the BBC wesbite about working the system to buy cheaper tickets - might be of some use.