Collection of fares on
HoW‡ services does seem VERY hit and miss it has to be said. I travelled Frome - Weymouth earlier this year on the 17:47 and could not buy a ticket before boarding. Arriving at Weymouth still no ticket checks = no fare paid... Weymouth ticket office being closed I couldn't then pay my fare there either. I couldn't then make up I boarded a London Waterloo service from there and got to Waterloo where the barriers were wide open and no ticket checks either. Infact the Catering Trolley was more prominent on that service! I had a First Advance for that service though... Still not brilliant either way!
There are trains from Frome which are pretty much a very likely free ride if you wanted to play the dubious Fare Evasion Game (I won't name them!) But I purchase my ticket at an interchanging station as normally I connect into a different train. Or I already have my ticket having purchased ahead of time. Staff at Bath Spa are normally happy to sell a ticket without any question (Only a very small number of times has there been a slight problem) Bristol can be a bit more problematic and depends on who is working the barriers that day. Some staff are great and a very, very small number obstructively unhelpful!
The likelihood is that they'll have had to buy a ticket to get back onto the platforms later that day however....
Unfortunately if I was a gambling man... I wouldn't bet on that at all