I am not a health freak or any such thing, but I have to ask you one question concerning health. Can you drink tap water in London? I know you are advised not to drink anything but bottled water every time you go to a foreign country, but is it the same for industrialized countries as England is? Not to say that just because of that England is more worthy then some other country.
I have to say that judging by my experience the industrialization is not going to help you out with impure water source. It is always smart to drink bottled water where ever you go. Take USA for example, this is the most developed country in the world and you dont see those people drinking tap water. Whit all this terrorism talk now days I wouldnt be surprised that water sources are targeted as prime attack goals. So, I would not drink tap water if I was in your shoes. And dont think you can take some bottle water on any flights, either.