If ground rent is over £250pa outside of London and £1k pa within London, the lease falls within the Housing Act 1988 and is an Assured Shorthold Tenancy for the purposes of the act. In the event of breach of the lease in most leases (in certain circumstances), the Landlord can apply to the court for forfeiture BUT the court has the power to grant relief against forfeiture i.e. the power to prevent the lease being ended.
If a lease is an AST for the purposes of the HA1988, the court has NO power to grant relief and HAS to order that the lease is forfeited if certain conditions are met e.g. 3 months' rent outstanding for 3 months.
So for some lenders, this is not acceptable mortgage security, because if you breach the lease and the landlord applies to the court for forfeiture, provided the conditions are met then the court will have no option but to end the lease. So the lender would lose their money and, as you'd then be homeless as well, it's unlikely you'd have the money to pay off what's left on your mortgage.
This didn't used to be an issue when long leaseholds were granted for peppercorn or low rents, but as landlords have got greedy this is an area of legislation which is is desperate need of review, as so many more leases are falling within the Act (and so many people don't realise).
Also, that rent is ridiculous.
If you follow the statutory lease extension procedure then the rent will reduce to a peppercorn and you will get 90 years added to the residue of the lease. You would be responsible for paying your costs and the landlord's costs (including solicitors' and valuer's fees) as well as any premium chargeable for the extension. This can be extremely costly so isn't an ideal solution by any means.