Hello,
When I go to the UK in May, I plan on taking trains to York, Bath, etc... for day trips and was planning on purchasing tickets via NationalRail.co.uk using a Two Together Pass. (I'm assuming this is the only/best place to purchase train tickets via the internet)
A friend, who just went to the UK a year ago, warned me that he bought train tickets only to find out that he didn't have a seat reserved. I guess I'm a little confused why that was. He couldn't explain it either.