It really depends what FC can offer, and that in turn depends on what sort of trains you're running and what route you're running them on.
So the ScotRail 'intercity' trains i.e. Glasgow-Edinburgh via Falkirk, Glasgow-Aberdeen, Edinburgh-Aberdeen, Edinburgh/Glasgow-Inverness all probably should, and should be more than just a single 170. Upon their being able to be cascaded, 220s or 180s would be more appropriate. Same for the XC routes using 222s, and TPE.
The problem is that there's a lot of overlap in service types. For instance, Birmingham-Stansted is an inter-regional express, but serves as a 'local' train between many areas. Were it stripped down on calling pattern it'd be worth having FC and some catering, but right at the moment it's pointless, and an all-standard 3-car 170 could easily do the job. The same's true for EMT's Norwich-Liverpool service, which probably should have FC, but a reduced calling pattern, as it's an intercity express train.
Finally, TPE is the same. I think the best option would be turning the Manchester Airport trains over to all-standard class, with extra luggage space, and running 'semi fast' e.g. calling at Stalybridge, Dewsbury and Garforth. Equaly I think there should be TPE services running 'fast' on a very limited calling pattern, with FC, full catering, end doors and the lot. Maybe the Liverpool-Newcastle 'express' of St Helens, Manchester, Huddersfield, Leeds, York, Darlington.