I joined the MGCC and am a member of the local MG club which is affiliated to the MGCC.
The MGCC caters for all MG owners, whther you car was built this century or the last. I would probably say the MG clubs have members who are more mature in years than the Mini owners I know in their clubs, but I am equally comfortable with both types of car owner. Certainly, being an owner of any club brings advantages (as others have said) for things like technical expertise, discounts and general fun.
MG club members, from my experience, seem to be more focused on maintaining their cars in original condition, whereas Mini owners seem to like modifying their cars, as well as keeping preserved originals. That, in my opinion, mirrors how the owners looked after them when they were 'current'...
As for rusty BGT's, you'll find most of these cars are in better nick than your F/TF so don't be so quick to pigeon-hole classic MGs or their owners! If your stereotypical vision of a classics club puts you off, maybe better looking for a boy-racer outfit...... (just joking!)
In summary, the MGCC is the main 'factory' owners club but has many regional 'centres' and they will be able to tell you who your local contacts are. It is certainly worth the ÂŁ36 annual fee.
Another benefit is that you can usually get a club stand entry for a show that might otherwise be hard to get as an individual entrant