A word of warning:
Last year, at a flying instructor refresher course, the former CAA legal enforcer Gordon Sharp was asked whether it was legal to fly the London heliroutes in a single-engined helicopter.
He sucked his teeth and said: "I don't know. There's been a lot of building since those heliroutes were established. It would all depend what the judge thought."
So there you have it. If you have a problem and fail to "alight clear" (duh?) you're likely to be prosecuted and your fate would hinge on a roll of the dice in court.
Official heliroute rules I don't know. Official arse-covering and stable-door shutting procedures I can take a stab at.