Appartments (flats) are usually owned leasehold. The buyer owns a lease to live in the property for a set number of years (99 to 999) after which it reverts to the freeholder.
The owner of the building is a person or company who owns the freehold and you will have to pay them an annual ground rent and an additional service charge for the maintenance of the building, building insurance and cleaning. Be sure to check the details of the lease when you view the property.

If the lease is soon to be terminated, it's now possible to renew it by buying it again from the freeholder.
Also if you plan to buy the freehold later, be sure that you know who is the freeholder as sometimes, the original freeholder has sold the lease to a develloment company which has itself sold it to other develloment companies.

Be aware of the amount of work needed to repair a structural problem. The fees are usually split between the leaseholders but you may end up to pay £25000 suddenly.