Remember that you must comply with Rule 5!
Even if absolved from the '1000 ft' aspect by an ATC requirement, you must always observe the 'glide clear' requirement. 'Green areas' in the middle of London (e.g. Hyde Park) don't count!
Met someone who once had an engine failure in a SE aeroplane over London - at night! But it was when airways didn't exist above 11000 ft and he managed to get his Attacker clear of London and relit the engine before scraping into Ford or somewhere.