If you have owned your property for two years or more, then you generally have a legal right to extend your lease under the Leasehold Reform Housing and Urban Development Act 1993. This means that the freeholder cannot usually refuse to extend the lease.
Difficulty may still arise if the landlord does not agree to the premium payable for the extension or the amount of their reasonable legal costs, which you will be required to pay. A dispute could be resolved by negotiation or mediation. If it is not, then it is open to you to ask a tribunal, namely the First Tier Property Tribunal, to hear the case and make a ruling as to the amount you should pay.