I dont think that there is a limit on mag for scopes from America to Uk, but there is a restriction on ballistic or military rets.
The UK doesn;t export scopes with anything higher than 4x mag (I think thats the mag off the top of my head). That might be what one of the above posts is thinking of.
I have had a few bits including a scope brought back, but you do have to be careful what you do. My daughter was moving around the states for a few months when she was at a college in L.A. and her boyfriend was working in Atlanta. They both shoot a bit so were quite keen on visiting and shopping at Bass Pro etc.