Stored routes can be updated whenever the FMC database is updated (usually on a 3-4 week cycle). The stored routes will be arranged by the airline and input by the database provider. My company (40+ aircraft) do not use stored routes and have not done so for 6 years. Special updates can arrive outside of the update cycle.
Entering routes manually isn't particularly onerous and it consumes about three minutes at the start of a flight. What usually is a problem after an airspace change is that new waypoints are not in the database and, as you point out, every such waypoint has to be entered as latitude and longitude and that means when we're given direct to somewhere that isn't stored, we may have to ask for the lat and long and that consumes radio times and it takes us longer to turn.
The problem would be greater with prnav arrivals and departures but I don't know any any (yet) in the London area.