Far cheaper, and just as easy, to bring your own food and drink for the train
Do you make you own bread and clothes too?

At the end of the day you make you choice and pay your money. Personally, I'm now old enough* and reasonably financially stable enough* to throw money at
some problems to save time and improve my quality of life.
I know some people who will drive many miles to visit a couple of zoos, drive home, and drive back to the same area the next day to visit a couple more zoos (then drive home again) because they've calculated it's cheaper than getting overnight accommodation. Personally, I'd be happy to pay a few quid more and avoid the extra driving but (I may have done the same when I was younger and) each to their own.
All that said, I think Zia's basically right -if you know how to play the game you can get rail journeys (even to London) at fairly reasonable prices** -I don't know how to play the game but my sister-in-law does and she seems to pick up some real bargains.
Feel Free To Ignore Below (too much lunch hour time to burn)
*Both of these took some time and a lot of hard work;
** Obviously essentially subsidised by the poor sod's*** who commute to London and buy very expensive season tickets;
*** One assumes there's some compensation to a central London salary whilst living in non-Central London (personally I'd rather have the commuting time for myself but, again, you pay your money and take your choice);
****Sorry for all the footnotes I'm reading Ayoade On Ayaode***** at the moment******
which does this and I think it's contagious*******;
*****It's good;
******Not literally at the moment, that'd be impractical;
********No doubt it's contagious.