In London, the longest stair set that is easily rideable without any hassle has to be the Greenwich Foot Tunnel – tight, slippery, steep. 100 stairs, I think it has no landings either. Send someone down first to check for walkers. Oh and start riding once the lift starts to move up from the bottom, as then the guy in the lift can't tell you off.
If you like jumping off stairs, then Canary Wharf has a decentish set. Also, I've seen the ones at Waterloo, from the bridge over Waterloo road down to ground level jumped down in quite a scary looking way, but they're not very long. Also the ones off Hungerford Bridge, but they are always a bit crowded, you have to wait for a decent gap.
Joe