But if you are flying over London within gliding distance of the Thames, and are fully prepared to accept the consequences of ditching there, in my opinion the ultimate objective of the "land clear" requirement has been met.
Well, watching the Thames from the terrace of the House of Commons the other evening I saw an endless stream of boats all over the place.
Yes you would probably have a reasonable chance of missing most of them ... but with no engine you limited choice of where to go and I don't think that you could guarantee that you could always miss all the boats.