Any offence that is fraud related is going to make life very difficult for you in future.
To answer the OPs question - you could go to prison, although it is unlikely. You WILL, however, be convicted of a fraud and fined heavily and made to pay a token gesture of compensation to the TOC (usually the fare you should have paid).
This can be accompanied with things like banning orders, ASBOs etc... depends on your attitude:
a) when stopped at the time
b) previously - any previous dubious involvement with the railway
c) in court - how you appear to the magistrate
d) when communicating with the TOC/their lawyers - whether you are remorseful
If you gave correct details first time of asking, then as I've said before, expect a fine over £500-£1000.
If you, in addition to committing the original offences, gave FALSE details or MISLEADING details then you may find you are fined over £1000 (multiple offences) and given some sort of community order.
Travelling outside of the European Area may also become difficult, if impossible.