LAA - as my Champ is on a Permit. I have been a PFA/LAA member since the late 70's, although I was mostly flying the usual rental aircraft types for the first 20 years or so. I supported it's objectives then, and still do. If it was not for the LAA permit route, I doubt I would be flying today; and certainly not the 75 hrs I flew last year!
Other than that, just the VAC - Vintage Aircraft Club, as the Champ is, well ..... vintage.
Sometimes I feel guilty about not being a member of AOPA UK, but overall I remain unconvinced that it's for me.