Apologies for asking basic questions, but I'm shortly visiting the UK. Haven't been to your nations for a while.
From the chat above, it seems conductors/guards have a handheld device for scanning tickets, and paper tickets may also be in use.
If staff sell tickets on board, are they printed from the handheld, or do they write a paper ticket? Do they accept Visa/Mastercard (depending on reception!) or cash? Do they give change? Do they have a laminated sheet or small book from which to calculate fares between station X and station Y?
And the last one for me specificially: I'll travel with a 1st class Eurailpass. The latter's Global Passes have been valid in UK since 2019, but am I going to have endless discussions with staff who haven't seen one and who incorrectly believe I'm trying to (as we say in Australia "pull a swifty" over them? (i.e. mislead/lie to them).