According to British media many ATM’s will be unavailable during the Olympics and the eight ATM’s that VISA provides, which only accept VISA cards, will not be enough to let every visitor withdraw cash.
Only use ATM’s in trusted areas and if possible, try to make any withdrawals from ATM’s located inside banks, thus minimising the risk of skimming. Both a conversion fee, normally up to 2%, and a withdrawal fee will often be levied.
In other words; buy British Pounds before you leave for London. If you want to use a VISA card, linked to your account, when paying, you can buy a FOREX Global Cash card, which is a VISA card loaded with a desired amount of money, which can also be used for purchases inside the arenas.