can I pay in Euro in London and do I need an adapter for electrical sockets and which voltage do they have ?
A: in some of the major shops you can pay in Euro, but most only take sterling. Yes, you will also probably need to bring an adaptor (not sure where you will be travelling from?) as the continental Europe and US plugs are different in shape.
I think the voltage is, however, the same as my UK electrical stuff always works fine with an adaptor in those places.