With most train journeys, 'First Class' simply gets you bigger seats (typically with 3 seats across the width of a carriage, instead of 4) which might be marginally more comfortable than those in 'Standard'. Some rail companies might also give you a free cup of coffee (perhaps with a Danish pastry, or similar) but that's usually the only difference (except, of course, that 'First Class' carriages, outside of commuter periods, are far less crowded).
As Hc4361 suggests though, it's best to check what each company offers. (Greater Anglia, for example, basically follow the model I've given above but they also offer free wi-fi to First Class customers, rather than charging a hefty fee for it in 'Standard')