OP, Im a bit concerned that you dont seem to know (and no one here can tell you) under what legal context you own your home.
You dont seem to know why you are a leaseholder, this should be something that was explained to you when buying.
You dont seem to know what the implications of leasehold are, very bluntly (and its not necessarily that it pans out this way) technically, at the end of the lease, if the lease ends, the property reverts to the ownership of the freeholder. So you are a leaseholder, which means youve leased the land the property is on. You own the property, but not the land it sits on. Very common for flats. Less common for houses
Im not aware enough of SO and whether there has to be a lease in order to create a rental arrangement for the portion you dont own, perhaps thats why there is a lease. It may be like you say that once you own the entire property, the lease ceases
Lenders of course dont like short leases because of that technicality above, that as time approaches the end of the lease, the more likelihood of the property reverting to the landlord's ownership. It might be 76 years away now but as time goes on, then its 50 years away, then 20, etc, and if you bought it with 20 years left on the lease and then died with mortgage outstanding or couldnt pay the mortgate, the lender wants to get a property re possessed that they can sell on. They wouldnt be able to with a 20 year lease, or even 50 year (easily anyway)
I think you need to do some investigations next week, get all your paperwork out from when you bought it.