Properties with leases under 70 years have in recent years become increasingly difficult to mortgage, and thus problematical to sell.
The calculations used to arrive at the value necessary to extend a lease is very complicated, but published. Further information is available on the government
lease-advice site. It's called a marriage value.
I think I am right in saying that the increase in property values over recent years has had an effect on the marriage value. It therefore follows that if property values decline (as I am confident they will for the next couple of years) then this may well reduce the marriage value.
I would suggest that increasing the term to 99 years is worth consideration. Increasing to 125 years is probably not worthwhile, as the value of the property if on the market will be unlikely to reflect the extra premium paid.