The usual £5 will be offered for a simple guide (of less than half a page) as to which carrages (types of trains? Routes? Types of Liveries? Types of operators?) on the entire GTR empire where standard class ticket holders may sit in first class (hints: Sutton/Rainham/387s)...!
Not sure my reply qualifies for the £5 prize, but here's a quick quote from the Southern website.
'A number of stations are served by trains operating on different routes, some with and some without first class. We strongly recommend that you check in advance whether your planned service operates with first class or not. The easiest way to check is through our timetables pages'
You can enjoy the full guidance here:
It's clear that the GTR empire runs an unhelpful ticketing policy relating not just to First Class, but also to brands.
This results in needless confusion, which must sometimes lead to harsh treatment of passengers. The Gatwick Express question has been much discussed, but those in the Lewes area will often find Gatwick Express stock turning up, with platform displays reassuring them that it's OK to board, as it's a Southern service.
I am a firm believer in the option of First Class.
Regarding Thameslink stock in particular, as has been mentioned upthread, First Class does offer superior facilities to Standard, and any passengers aiming to enjoy it (declassified or not) must make the effort to go to the extreme ends of the train, so it's unlikely that many will find themselves there by accident.
It does seem possible that the individual concerned in this incident suffers even more from cockiness than from autism.
Nevertheless, over-zealous enforcement of over-complicated rules doesn't do 'The Railway' any favours.