Betterfromabove
From your profile you are PPL(A) so no-one here is going to give you a hard time for asking questions about heli operations. If you want to find out a little more about what helicopters can do, I suggest that you post in the "Spare Seats" in GA and try to get a ride. It would not take long to show you how (for example) a single can be flown "no hydraulics". I fly singles, and frequently practice both flying and landing without hydraulics just in case I ever need to. I can also fly without the tail rotor (landing gets more difficult but I can still safely leave London without it, if I have to) and again I practice this regularly. Finally, we regularly practice "engine-off" landings just to keep sharp.
If you are very lucky, even more lucky than just to get a seat, you may even get a flight through the heliroutes. If you do you would see how many options there are for landing in the remote chance that the donk stops. Include in this potential landing area the Thames, as any single undertaking passenger work has to be able to land there in an emergency.
The size of gap needed to put a heli down without an engine varies with the size of the helicopter; but a large machine is likely to be multi-engined and a multi is more likely to be able to leave the area on remaining engine(s) to land at one of the many airfields surrounding London. For a single the size of an EC120 (5-seat single turbine, 1.7ton) the area required for a forced landing is about the size of the penalty zone on a football field (though more is always nice!). In a forced landing we don't have the forward speed of an aeroplane.
Regards
John