Not how, where: international waters. London, IIRC, is not very distant from those. (Of course, London being a helicopter no-flight zone - for decades now - complicates things somewhat)

Edit: Helicopters specifically; but with the ground-to-air missile sites installed around the Greater London area (ostensibly for protecting the Sporting-Event-That-Shall-Not-Be-Named; a WTF in and of itself), I'd be very careful in mucking around the British airspace: http://everything2.com/title/crashing+a+helicopter+in+Centra...

Is it just no flight for helicopters? How about blimps? There's really got to be a solution here. It's a technical problem and we're technical people.

In general getting permission from ATC for flights over London is non-trivial (primarily because it is sufficiently densely populated that any emergency, especially at low altitude, is likely going to end badly).

You could tether a blimp. I just saw it done a day before yesterday night here.

London's definitely not a no-helicopter zone - I see police and media helicopters over my house all the time. You can get what's known as SVFR clearance (generally below 1000ft), see http://www.pprune.org/archive/index.php/t-68430.html

Getting 12 miles into the North Sea or English Channel without getting intercepted is a non-trivial undertaking.