Congested Area Min Alt
I've only been over the city of London twice, so I can't comment much about it as I'm not that knowledgable about the UK rules of the air.
In Ireland we are always supposed to be at min 1500ft amsl over congested areas, but quite frequently I am told by ATC to fly "not above 1000" to facilitate commercial traffic into the airport or if the police heli is nearby (you need to give those low time air corps pilots lots of room!) now before all the air law brains come on with the "your the commander tell atc you cannot comply" lark, the alternative is to be re routed half away around the country and economics and common sense dictate that you facilitate atc as long as it does not pose a risk to your flight.
In my opinion, when overflying a really congested area you have few options if the engine fails (in dublin my first choice would be the river liffey which runs through the centre, the VFR route for helis is along it) so whether your at 1000 or 1500 or even 800 won't make that much difference besides you maybe getting a mayday call out.
I'd like to have a second engine always too, but if I owned a news corporation, I don't know how I would justify a 355 when the 44 will do the same job, for an eight of the price.