Next weekend I will be in London for the first time for almost a day. I am planning on taking the Tube from LHR to downtown and spend the day sight seeing and then back to LHR (I am staying at the Renissance at LHR) for a morning flight the next day. Will I need cash or will I be ok with a MasterCard? The only thing I could think of was tipping the bellman at the hotel (if even necessary) - can I tip him in US dollars or will that annoy them?? Is everything else Credit Card friendly?? I will have US Dollars on me, but if I don't need to convert them, I would not waste the expense.

Thanks

Josh