"almost" is doing a lot of heavy lifting here...
While the majority of flats are leasehold, by far the vast majority of property in England is freehold. Only a fifth is leasehold.
While technically a leasehold has a fixed term and at the end of the lease (usually starts at 99 years) the land owner technically owns the property, in reality this scares most people so usually when someone sells the property (usually while there's still at least 80 years left on the lease) you try to extend the lease again back to 90 years. So while it is possible for the lease to run out and people lose their property, it's usually something you'd be expecting when you took over the lease (and so you'd pay a correspondingly lower price for the property). While the lease is active, there's usually an annual fee from between 100 and 10000 pounds. Obviously, the higher this is, the lower the sale price of the property is likely to be.
Personally, I wouldn't touch leasehold with a bargepole, and unless you want to live in the centre of a city there's usually plenty of freehold property available so you don't need to go down the leasehold route.