It is somewhat of a paradox that if you have more than 80 years remaining on your lease you are unlikely to feel the need for a lease extension. Indeed may lenders and conveyancing solicitors alike are happy with leases of 70 years plus.
That being said the best time for lease extension from a cost perspective is when a lease has more than 80 years remaining. If you own a flat with just 80 years remaining on the lease, ignoring that fact could cost you dearly. 80 is the magic number of years, below which it more expensive it is to buy a lease extension from the owner of the freehold reversion.
If you have more than 80 years left, the price of purchasing a lease extension is ordinarily small and the ground rent reverts to zero. You have to do it to stop the flat depreciating in value. In effect you are compensating the landlord for (him or her) not getting your flat back. If you have more than 80 years left, there's not much to compensate the landlord for.