I have flown around London three times now, all in a DA-40 with dual Garmin 430s. Although I normally try to navigate by sight, map, compass and stopwatch, with the occasional VOR thrown in over featureless/unfamiliar terrain, I find that around London a moving map GPS is invaluable, particularly if you're not familiar with the area.
I'm now planning a flight in that general area in an aircraft which has no moving map GPS, no ADF and no DME. I'm already hunting for a handheld moving map GPS to buy or borrow.
This is one of the places where you can into trouble fast despite Proper Planning so make sure everything is in your favor.