Collectively buying the Freehold of your block has been a legal right to qualifying leaseholders* since 1993. Leases will continue to get shorter and collective enfranchisement is one solution, if you cannot get the requisite majority on board claiming your statutory right for a lease extension would be another.
Owning a flat only gives you rights of occupancy for the term of the lease. When your lease eventually expires your flat reverts to the Freeholder.
If you own a share in the freehold then at the end of your lease the flat reverts to the Freehold owning company not to you individually. But, normally most Freehold Companys have a board meeting and vote that all shareholders be allowed to extend their leases to 999 year leases. This should be done via a formal Board resolution to "permit any lessee with a share in the Freehold to extend their leases at Nil premium".
Sometimes granting new 999 year leases is executed at the time of the actual freehold purchase transaction for convenience, but it need not be so. If you need to keep costs down this can be done later. After all a short lease is only a problem when perhaps you are selling or remortgaging and a mortgage lender has a policy that they will not advance the normal loan to value ratio if the lease has say less than 60 years unexpired.
Increasingly legislation has been changing to benefit the leaseholder who want to enfranchise. For example qualification criteria have been relaxed and it is no longer necessary to live in the flat.
Exercising the right requires a formal legal notice to be served on the Freeholder and all intermediate landlords.
Here the Leasehold Guidance Service provides a step-by-step guide to Buying Your Freehold.