PPL's can navigate over long distances visually. As long as you keep on top of the navigation, then there should be no problem.
The area around London is fairly congested, but it shouldn't be any problem to anybody who's been taught properly and is in current practise.
Unfortunately, this doesn't seem to be happening. On my last instructor revalidation I was told that the new CAA bug bear is PPL navigation, mostly due to the findings from OnTrack. It is something that is a problem we find on a regular basis. Navigation is a skill like any other. If you don't practise you will be rusty. Since most PPL's stay in the local area comfort zone, their nav tails off markedly over time. The concentration required if you aren't confident is very tiring and I would suggest that people build up the length of their stages. You could of course do a lands end to John o'Groats trip the day after passing your test (As long as the CAA speed up their licence issuing that is!), but it isn't really advisable to do it on your own first time.
You are doing exactly the right thing GTW in asking on another thread for some advice. I wish more people would do that rather than feeling silly and not asking as it would be an ego blow. Nobody cares what questions you ask, but we will care if you go blundering into the LTMA!