It's cheap to extend a lease if there are > 80 years left, maybe £2,000-3,000. That's just legal fees and ground rent foregone. After that it gets expensive as you have to share the increase in value caused by the new, longer, lease with the freeholder.
Leasehold is evil but commonhold isn't much better, which is why hardly any have been set up. It's difficult to get every owner to cough up for repairs. You'd have to sue them; they can't be evicted.