I don't know if it used to be derogatory... but now I believe it's just kind of slipped into common parlance.
I've just spent quite a bit of time in Thailand drinking with Aussy guys who referred to Brits as poms every few minutes - there was nothing uncomfortable about it at all.
I think the US equivilent is limey?
Limey is commonly thought of as being a derogatory term. Brit isn't. Kind of odd to me that the British Navy was smart enough to take measures that prevent scurvy and those measures somehow became a derogatory term. Jealousy I guess.