More often than not, if you're buying a flat in a modern complex or in a converted house, you're going to be buying on a leasehold rather than a freehold basis.
The most important thing to take into account when weighing up the pro's and con's of buying a leasehold or freehold property, is that when you buy a leasehold flat or house, you only get a right to occupy the pad for the length of the lease. So, a 99 year lease will allow you to reside in the flat for your whole life. Watch out for those houses on the market with a 5 year lease!
The most common reason for a property to be sold on a leasehold is down to the fact that you might be buying a flat which is part of a larger building which is divided up into other flats which are separately owned. Leasehold properties are not uncommon in the UK and you can expect to find 10's of 1000's on Adzuna.