I'd love to beta. I'm experienced, I'm an English major who loooves grammar (nerdy, I know). American, unfortunately, so I won't be much help with Brit Picking, but will do my best. My favorite pairings are listed below; I am willing to do whatever pairing is needed, even *gasp* het. Femmeslash is fine as well, what with being gay and all. ;D
Favorite Pairings: Harry/Draco, Harry/Severus, Harry/Lucius, Harry/Charlie, Harry/Ron. Really Harry/whoever, as long as Harry bottoms. :D
Squicks*: top!Harry, major character death
Would-be squicks that I don't have a problem with: BDSM (love it, actually), Dub/Non-con, minor character death, NC-17, crack!fic, OOC, AU, crossdressing.. pretty much whatever.
That all may be way more information than you were looking for.. oh well.
*That's not to say that I won't beta those types of fics, I just tend skip over them when looking for something new to read. As a beta, I don't really mind.