..however if you are flying (a helicopter) in the London CTR and are unable to maintain sight of the surface or the required 1,000m in-flight visibility, would the correct course of action be to immediately declare an emergency and climb to a safe altitude (MSA albeit in IMC)?
Eastbound along the Thames, LHR would have a fit in case you conflicted with approach traffic.
Or if you were in sight of the surface (and presumably a suitable landing location), would you not try to put it down - even landing on the water if necessary?
Yes this would be the thing to do in an extreme situation.