Full membership requires two things. Member of MGOC and owning a V8
See here https://www.two-sixties.co.uk/mgoc26...141-Membership
If you lost the form just PM Tim and he'll send another one.
With regards to the brakes, skimming the rear discs is cheaper than replacing and won't affect the handbrake.
As the handbrake is a drum brake inside the disc it won't be part of the normal skimming process.
If due to excess rust people decide to turn the inside of the drum the change in diameter may give problems with the handbrake shoes (not enough travel to touch the drum).
New discs via the shop are good value for money given the numbers of years the rear brakes should last.
Always good to have a spare set incase there's not enough meat left to skim.