I believe the general consensus is that in a small plane there is a roughly 50% chance of surviving a ditching unscathed(short term, long term depends a lot on circumstances such as Thames vs. North Sea etc.), which for me means that it can never be the primary option in case of forced landing.
I wouldn't want to do it, either. If you fly along the Thames along the official heli-routes, you are deemed to have accepted the requirement to ditch between the high and low water marks, in the unfortunate event. If the tide's in at the time of your forced landing, tough luck. I see no reason for any other forced landing place being deemed acceptable by the CAA, especially because it's a fixed wing.
IIRC, the entire Heathrow CTR has fairly recently been deemed a congested area for the purposes of the "Glide clear" rule for single engined fixed wing. So the CAA are onto the case, as it were, even though you wouldn't be entering that airspace, by the sound of it.