Ryanair have employed American Pilots, and I stand to be corrected on this, but i believe Aer Lingus have a least one Aussie flying for them
BA have Aussies, Kiwis, Americans, Canadians, French, you name it flying for them on mainline. All these people have the legal right to work in the UK (in any job). The issue at stake is that US pilots without the normal right to work in the UK are flying wet-leased aircraft on the N register which would otherwise be G-reg and operated by British (EU) pilots. Funnily enough, I can't imagine the US allowing any British pilots to do the reverse....
This is the reason GSS exists in the first place as Atlas tried this on a few years ago...