Your question doesn't make a lot of sense. Buying the leasehold of a leasehold flat doesn't make it freehold, it makes you the leaseholder. It is quite common for the lease to contain clauses preventing sub-letting. If you were somehow to buy the freehold as well it may be possible to remove this provision, depending on whether it is a term of the lease or a covenant, and depending on the reasons behind the clause/who the beneficiary is stated as.