US supplies 115V at the output to the plugs so about half what is provided here. When you buy a US to UK adaptor, it should be more than just a pin converter - it should also have a voltage converter - most usually a step-up transformer. You will be able to tell if the converter you bought is one - they weigh a kilogramme or so. Mine has a UK three pin socket on the plastic box, inside which is the transformer, and a flying lead with a US plug that goes into the US socket. It delivers about 60W.