I wouldn't write it out. Every character has an accent, it's just that you're used to characters speaking with your accent so you haven't noticed. With RP being international, chances are you are going to be playing off a person from that part of the world, and they will make faces at how you are writing out their accent.

Rather than looking at transcribing how you hear the accent, listen to ways that they say things, like the idioms or culturally unique turns of phrase. You'll convey a lot more by mimicking that than you will dropping "gs" and all that. Or, drop an OOC note in the first paragraph or if your game has a stats and permissions section note it there.