8ball.co.uk has a couple ok things, but it's mainly the better known underground metal, otherwise it's a site catering for mainstream metal.
I'm obsessing at the moment over displeasedrecords.com They're based in holland but they accept british notes as payment, fast delivery, good service, and they've got a really good selection. quite a few obscure items.
I like displeased records. =)
(hey, they released None So Vile, how much more respect could a metal label have?!)
[edit] PS, awesome prices too of course! At least half of the CD albums in their catalogue are under a tenner!
[edit no.2] PPS. Two other half decent UK based mailorders are musica.co.uk and cow.co.uk They're CDs only though.