Leasehold is a more common type of purchase since a flat is usually owned as separate property. The neighbouring apartment owns their own space with the common areas maintained by the freehold owner in return for maintenance fees.
Having one freeholder for an apartment block is less risky from a lender perspective since it avoids splitting the liability or responsibility for upkeep.
Mortgage rates will depend on the lease period - a leasehold with over 90 years to run is an easier prospect than one with 60 years or less.