Well,
A leasehold is a lease from a third party, generally the company who built the building.
You may them ground rent, which is generally 40 or 50 pounds a year, and service charge, which can be a few hundred a year - you've got no real control over that. If the owner neglects the property or constantly overcharges the service charge, you've got no real recourse
Adding that the the 99 year thing, this isn't a very good deal. So you have the right, if a certain percentage of the leaseholders in the building agree, to buy the freehold. You then, between you, manage all the maintenance, etc. And there is no expiry date.
I guess it's a bit like a housing cooperative.