If it can fulfill requirements for sec 58 but you want to shoot it, so it goes as 7.1 or 7.3
if it is unique, historical, of technical interest or just very rare.
this explains in a bit more detail...
http://www.bedfordgunclub.co.uk/heritage.html
the above gun is of historical interest because it was part of the trials held by the MoD when they replaced the Hi-power with the Glock 17, so you have to show to the Police / Home Office what your collection is, and why the gun fits it.
no-one would really give me much advice TBH
next up, I have my eye on another trials gun, but I also would like a webley 455 of some sort too