hmmm i reckon you'd be hard pressed to get a decent semi for that money, possibly a hatsan, but more likely a second hand mossberg, i recall a new one is around £450 but IMHO you should spend a bit more and get a decent second hand Beretta, or lanber, basically something that won't blow up in your face.... I use a cogswell and harrison s/s ejector, a lovely gun and it has so much.... not sure how to describe it, but it was made around 1920 and is just really nich to hold and use....
i'd go to your local gunshop and see what they've got. i'd really look at spending at least £600 to get so