"Seal kit" is just a pile of O-rings.
If your mate is willing to service his gun himself (no biggie) he can take it apart, measure the o-rings, and buy them individually almost certainly for far less than any "kit" will cost.
The ones I had some difficulty in sourcing - but not in the UK - were the x1 (1mm) thick ones that lie behind the brass exhaust valve seat. Simply because no one local to me stocked x1 o-rings. In the end I paid a bit more for these by buying an RC car engine service kit. But I'm sure that in the UK you can source these no prob.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS