I'm not so sure. As I mentioned on
this post in the Megatrain topic as far as I can determine the instructions for staff for break of journey when not entitled to are as follows:
"Charge the difference between the fare paid and
the appropriate Single fare for the overall journey
being made that allows break of journey."
That's
not the same as "Charge for a new full Single fare for the journey being made" and is likely to be significantly different in this case (it was only £1 difference in the Megatrain case).
My source for the above quote is the East Coast revenue manual. However although this
doesn't speak for FGW,
East Coast appear to have a policy of letting customers out of stations without charge, as we found out when Professor Evans' unpaid fare notice issued at Durham station in September 2010 was cancelled (I also was informed, via PM, by EC gateline staff who expressed surprised that the Professor was issued the UPFN, as they informed me they were instructed to let people out in such circumstances, but with a warning that if they tried to resume their journey they'd not be able to resume on that ticket).
(We also know that LU will let people out at intermediate stations on cross-London transfers, but the transfer is considered expired once out of the system and a new ticket would need to be bought to resume on LU. This obviously does not speak for FGW though).
Finally, lying to staff is not advised or condoned.