Hmmm, he may have no choice. Strict liability offence and all that. But, it shouldn't go to Court unless the railway can demonstrate the customer was told about the Groupsave restriction - which SWT now do.
You have said this a few times before, but can I ask where it comes from as I'm trying to get to the bottom of it?
I'm not doubting your sources at all, especially they are likely to be more up to date if you work with the revenue and security team at XC, but the DfT website says that prosecutions for fare evasion are carried out under the Regulation of Railways Act 1889 ("Before penalty fares were introduced, the only way to deter people from travelling without a ticket was to prosecute them under the Regulation of Railways Act 1889"), and the CPS website says that it would expect prosecutions under the same Act (although this does not mean anything really).
This Google search also reveals many results from forums and other pages with people and Tocs posting that they are being prosecuted/carrying out prosecutions under the 1889 Act (which does require intent).
Saying that, having read the Byelaws I cannot see any technical reason why prosecutions under fare evasion could not take place under them. Perhaps it's taken the railways several decades to realise they can, or there is another reason hidden in some other legislation somewhere which says they cannot.
Rules governing penalty fares and byelaws on London Underground are different to those on the Tocs, but the 1889 Act also applies to fare evasion.
TfL's Revenue Enforcement and Prosecutions Policy said:
Fare evasion on London Underground is contrary to the Regulation of Railways Act 1889, and the London Regional Transport Railways Byelaws (LRT Byelaws).
This is the same as on the Tocs (except obviously the railway's own Byelaws apply and not the TfL ones.
In addition, TfL will prosecute the following offences (cut down to remove fraud, etc offences for which we are not concerned with).
TfL's Revenue Enforcement and Prosecutions Policy said:
(a) Fare evasion in contravention of the Regulations of Railways Act 1889
(b) Contraventions of the London Regional Transport Railways Byelaws
I am not sure if this means that they only prosecute fare evasion under the 1889 Act, and that other non ticketing Byelaw contraventions will be prosecuted under the Byelaws. Even if so, there must be a reason why the they would bother to use the 1889 Act at all, given a prosecution under it is harder than a byelaw one.