I wondered this... we went out the other night and came across a club; asked the entry fee and were told £1.50. So we went in, but when our male friends caught up and came in a few minutes later (they'd been seeing another friend home safely) they got charged £3. Turns out it was ladies half price.
I didn't realise this was allowed! But I suppose when you think about it it probably is... I mean, look at student discounts or freebies. That's definitely allowed but technically someone could say it's discrimination against non-students. I know that it's very different to gender, but still.
Not that I'm saying it's right that they do this; it's not fair on guys. And to be totally honest I'm not sure it even works that well as a tactic to 'attract girls'. But I think it's probably legal. I'm no expert though so who knows!
EDIT: Why am I being negged so much for this?! I don't think I said anything particularly controversial, everything was pretty much opinion based, and I might be wrong about whether it's legal but I clearly stated that I don't KNOW it's just a guess...