In the days of loco-hauled trains, first class would always be at the "London End". But these are gradually getting replaced by multiple units, and first tends to get placed in other locations, for various reasons (e.g. cabin size, engine location - often underfloor, catering facilities, etc, etc). Where you've got a longer train, you'll typically get some indication as to where to stand on the platform if it's a through station.
If in doubt, the traditional marker for first class is to have a yellow stripe painted along the carriage, just at the top of window height. Once you see it, it becomes obvious.
Do be wary of the "staff carriage" on Virgin West Coast, though
