Out of curiosity I decided to see how much a three hour train ride would cost. The route Brighton-Oxford avoiding London takes just under this amount, and costs £20 return with a railcard. I'd imagine a season ticket would work out to be around the same price, and if your travel route included London the price would jump. So, that's £100 per week spent on travel (days off? Unlikely, I'm afraid), which is more or less the same price as university accommodation.
That is certainly not your only expense though, since you won't be able to go home to eat lunch, that could add an extra £3-5 a day, depending on your tastes. Add this all on to the fact that you have got to spend six hours every day travelling to and fro' and you have a frankly abominable university plan.
You will not have a social life, at all. Where and when do you plan to do set assignments? Believe me, working on a train is neither comfortable nor desireable (and in my case, nauseating). If you do socialise on the weekends then your degree will suffer and you might as well not be going to uni at all.
You may say you don't care about missing out on uni experience now, but I'm more than willing to bet you'll change your mind later. It's unlikely you'll be able to enter uni accommodation halfway through your course, which may force you to walk out with a storm cloud over your head.
Also, as a side note, you don't have to pay rent during term holidays anyway, unless you apply to do so.