Presumably a very weird sex act. If it isn't, I'm sure that there all plenty of deviants prowling about there that will invent whatever the posh wellie is. I feel like it will have something to do with feet, seeing as a posh wellie is a nickname of sorts for Wellington Boots (I think) so it would make quite a bit of sense. It would also make sense for it to include rubbers, since Wellingtons are rubber boots. That's all I can think of, get to you glorious internet deviants.
Oh man, last night was a dream come true! We did the posh wellie for hours!