Have you not had a conversation with the Buyer about all the upcoming changes in this space?
I mean, my own personal advice to anyone who is contemplating taking any action with regards to Leases... extending or purchasing the Freehold reversion... would be to wait? The Government appear to have plans where this particular headache could, effectively, disappear. Now, the Buyer - admittedly - may be receiving pressure from their Mortgage supplier (there's not enough information to say) and, in that case, you'd have to push it through yourself, or willingly allow the sale to fall-through.
From
https://commonslibrary.parliament.uk/leasehold-reform-in-england-and-wales/...
"Leasehold reform was included in the Law Commission’s 13th Programme of Law Reform with the aim of finding ways to make buying a freehold or extending a lease “easier, faster, fairer and cheaper.”"
Unless there is pressing need, normal advice would be not to act now... your situation might be considered pressing need.