Just to confirm, yes, anyone can be a member. I believe that you have to be a resident / citizen in the UK if you want to stand for the board, though (although I can't find anything saying this in the AoA...?); otherwise you would be exactly the same as any other member.
I don't think there are any legal requirements in that regard, we made it a requirement for the board elections because it's impractical for someone not in the UK to sign forms, etc. I would probably vote against any candidate not usually resident in the UK for the same reasons, but I don't think there is anything to stop them standing.