Don't forget that the right to break your journey or cut short is in fact a right that we have in the NCoC. It is only with Advance tickets that this right is not available. That means you are, at worst, breaking an unenforceable condition of a ticket, that's hardly going to count as committing fraud! The mere suggestion of it is laughable. The TOC will no doubt be told that they are wasting court time in the incredibly unlikely event of them attempting it. Anyone who is unwell or had suffered a bereavement or if an emergency cropped up and they were forced on to Bristol would surely be able to sue due to their human rights being breached and they'd have a very good chance of success for unlawful imprisonment I'd say.
The NCoC states:
"You may start, or break and resume, a journey (in either direction in the case of a return
ticket) at any intermediate station, as long as the ticket you hold is valid for the trains you
want to use. You may also end your journey (in either direction in the case of a return
ticket) before the destination shown on the ticket. However, these rights may not apply to
some types of tickets for which a break of journey is prohibited, in which case the relevant
Train Companies will make this clear in their notices and other publications.
If you start, break and resume, or end your journey at an intermediate station
when you are not entitled to do so, you will be liable to pay an excess fare. This excess
fare will be the difference between the price paid for the ticket you hold and the price of
the lowest priced ticket(s) available for immediate travel that would have entitled you to
start, break and resume, or end your journey at that station on the service(s) you have
used."
This clearly states that for certain tickets, the rights of break of journey, or ending short may not apply to certain types of ticket, in which case this will be made clear in the train companies notices and publications.
If you purchase the ticket from a booking office, you will be given a card which states that stopping short is not allowed.
If booking over the internet, the ticket conditions on the South West Trains website clearly state:
"You may not start, break and resume, or end your journey at any intermediate station except to change to/from connecting trains as shown on the ticket(s) or other valid travel itinerary."
This exact text also appears on the National Rail website, and to me seems to be clear.
To get out of the station, you would need to pay an excess fare. Those with FRPP access will be able to confirm what that would be.