Liz, if you wanted to collect rainwater from a London roof, there's a couple of things you'd need to watch for.

Pigeon poop is great fertiliser for all sorts of bugs and bacteria; it might be great for gardens but not for human consumption. For this reason, many in Australia have a 'first flush diverter' that allows such water to be diverted away from the main storage tanks.

Another is heavy metal contamination from either lead (in older houses) or zinc, in newer ones with galvanised steel. Normally, both metals don't get liberated into the water in quantities likely to do harm but, where you have acid rain, the metals get liberated.

A third thing to watch for is the effects of chimney flues on your water quality; you can get a fair amount of creosote on a roof downwind from the flue and the water is tainted something awful.

Cheers, Rowan