If you want a piece with history, I'd look at an Enfield P53 smoothbore. They range in price from £250 - £400.

Otherwise, as Lew07 states, a nice deactivated shotgun.

If you fancy a working shotgun but without a requiring a license, I have a nice old 12 bore pinfire side-by-side shotgun. Cased it's £475 (mahogany with brass circular drop handle). Without case, it's £295.

nick@njcreative.co.uk