You could try Hardy Browns in Sheffield - they stock some powders / primers etc. Closed Mondays from memory. Otherwise Countryman of Derby a bit further south.
Depends which side of Sheffield you are and when you need them but I've got 60x Federal 210 here (south side of Sheffield on edge of Peak District) you can have or about 180 Federal Premium Large Rifle Magnum (GM215M) and 150ish CCI Large Magnum 250 which I'd only want a few quid for if they are any good