Further comments having actually read the page:

Membership should be open to non-residents, especially if they have
some connection to the UK (non-resident UK citizen, for example). The
use of proxies means having non-resident members doesn't make it
harder to find a quorum. I also see no reason to restrict it to over
16s. Under 16s can't be on the board, but they can be members as far
as I'm aware (I've read the appropriate Act and didn't see anything in
there about age restrictions on members). I also don't know of a
requirement for a parent's signature, but it probably wouldn't hurt -
the Ł1 liability probably wouldn't be binding without one, but that's
just a nominal amount anyway.