This isn't just a UK thing. Well the 999-year lease is a UK/Commonwealth thing, but I've worked in NYC real estate, and a lot of projects have multiple hundred year leases.
And interestingly enough, there is a 99-year lease which the 99-year term was not literal, but merely an arbitrary time span beyond the life expectancy of any possible lessee (user) or lessor (owner). [1]
And interestingly enough, there is a 99-year lease which the 99-year term was not literal, but merely an arbitrary time span beyond the life expectancy of any possible lessee (user) or lessor (owner). [1]
1: https://en.wikipedia.org/wiki/99-year_lease