Even the older British guns tend to favour quality rather than size (ooer!). I've found that modern German pellets outshoot the older 5.6mm every time. Old Wasps, if you can find any are often the best out of the pellets, but even they are knocked into a cocked hat by either RWS Superdomes, Hobby, Superfield, or H&N pointed.
Not one of my British guns prefer British pellets over one or the other modern German offerings. That includes Diana 25 & 27, Webley MKIII, Hawks Mk1, 2,& 3; Osprey, Vulcan. BSA Meteors, Airsporter Centenial, the list goes on.
Nostalgia's a fine thing, but the only British pellet which even came close to any of the German offerings was the old Wasp
The South of England has 2 good things, the M1 and the A1. Both will take you to Yorkshire.