My system.

[Grumpy Grandpa]

I think you've already answered one of your questions in the description of your waterproofs. "Old thin waterproof coat and water proof trousers" versus, a "decent waterproof coat". You already know which is going to stand up best to a British summer!

Only you can best judge whether being "camo'd up" is necessary but as the Highlander has already said, the last thing you want is to attract attention.