A former colleague of mine commmutes to the smoke from Northamptonshire and goes through a couple of cheap folders a year. She's been using them for five years. She always pulls my leg for buying an expensive Brompton for £750 two years ago (It paid for itself in saved tube fares within nine months). She spends about £400 a year on cheap folders and on getting them fixed when they break. So, so far, enough for more than two brand new Bromptons.
I've not tried Dahons or Terns, but, aside from the omnipresent Bromptons (usually at least four on the 7.31 from Dorking each morning, with more boarding along the way), they're the ones you see more than once or twice. Buy a super cheap one and you'll find a couple of things:
1) The fold is not that small - a serious consideration on a commuter train that may already be crowded when you get on
2) They're as flexy as.
I'd strongly recommend you try a more expensive folder. I borrowed a demo Brompton from my local dealer, Cycleworks, for a week to give the whole thing a try. I had to put down a £200 deposit, which was returnable.
If you just buy a cheap folder to see what it's like, the chances are you will be put off. It really is a case of getting what you pay for, and a cheap 'un will robably put you off the whole idea.
My advice? Borrow or demo a couple of the spendier folders. You will get the money back in saved tube fares in no time.