Current UK law states that as long as you play three games (days) of airsoft at a proper airsoft site you then get the ability to buy airsoft guns that look the same as their real steel counterparts.
You can however buy those with different colours/ clear parts without any airsoft site participation, but they look bloody awful! Still quite a bit of investment in time and a few pounds to play, but quite fun nonetheless! Incidentally Northern Ireland is the only part of the UK were you can still legally buy real handguns, with the appropriate licence of course, go figure!
I'm not to sure with regards to law on this, but some of those airsoft forums have "used guns for sale" sections, the law regarding sale might simply be restricted to "new" airsoft guns, not second hand.