The best place to do currency conversion is at Thomas Exchange on Maddox Street in Mayfair. They are (as of June 4th) buying USD at 171.6 when the spot rate is 167.48, for a conversion cost of under 2.5%. If you convert over £5,000 the rate dips to 1.5%. Everywhere else is a complete rip off.
Private car service Wheely used to accept bitcoin, but recently removed the option. Uber works, but it is barely any faster and way more expensive than the underground.
Unfortunately, the UK is not supported yet for mobile phone top up at piiko.com, but perhaps by the time the conference rolls around they will be added to the 100+ countries supported.
https://www.piiko.com/static/mobile-recharge-coverageThe best way to avoid getting killed on roaming charges while you're here is to order a prepaid UK SIM card to your house before you come, and pop it into your phone once you get here. You can add credit on them easily. Examples:
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