I was planning to only use this journal to publishing stories, but I think I'm going to rethink it and write some notes pertaining to my stories as well... I got some really valuable tips on British English in my post to little_details. I'm starting a list here, but will add on to it with some words given to me by my dear insult-loving Australian friends.

arsehole - mild.
bastard - can be either affectionate or insulting, depending on tone.
bitch - "usually not forgiveable".
bloody - ultimate British emphasis.
cunt - apparently the worst insult ever in British English.
dyke - acceptable in some circles, an insult in others.
fat arsed - works great with "lazy" in front of it.
fucker - pretty universal, me thinks.
lesbo, lezzie, lez - degrading terms for lesbians.
prat - mild insult meaning "socially inept or incompetent person".
prick - insult, usually towards older guy full of himself, says erianea
swot - a know-it-all.
twat - moderately harsh insult, used in most dialects.
wanker - really rude insult, about guys.

positive words
fit - means "hot", may and may not be outdated, depending on who you ask.
gorgeous - less common, but possible.
hot - perfectly acceptable
lovely - instead of "hot" or "gorgeous".
pet - affectionate term.

bloke, lad - guy
bludge - ride along on other people's work (I think). Australian English, not British, says eiranea
daft - with your head in the clouds, says eiranea
dodgy - questionable.
get off with, pull, snog - making out.
mate - friend, possibly male only.
queer - may be used more than "dyke".
shag - actually used by people not named Austin Powers.