@nepaliketo30 - your advice is funny and inaccurate. You are just hitting in the dark. Little knowledge is dangerous.

@Donald_duck - Look at this link. There are advices and discussions.
http://www.tripadvisor.com/ShowTopic-g186338-i17-k2848787-How_easy_is_it_to_use_cash_US_dollars_in_London-London_England.html