Obviously, these tips are really just as good for any country, not just London and the U.K.
Money... Cash, Travelers' Cheques, ATM/bank machine cards, credit cards... Traditionally, travelers' cheques have been the secure answer to a travelers' needs. Banks and Exchange bureaus will exchange them for a fee if you can't get to an office of the issuer like Thomas Cook or American Express. And they're still a viable, and secure option.
I gave up TC's. I rely mainly on my ATM card and my credit card. I don't think my ATM card would work as a debit card outside of Canada, possibly the US but it's also the money machine card.
1. The ATM card will default to your PRIMARY ACCOUNT ONLY. For most people that is your CHEQUING account if you have that connected to your ATM card. At least that's been my experience. You very likely won't get a choice of what account to take your money out of. Make sure your PIN number is 4 digits, NO LETTERS Just numbers!
2. You can also get cash advances off your credit card. Make sure you know what your daily limit for cash advances is. Translate this into the local currency. 500 dollars a day is *not* 500 pounds. On credit card cash advances, interest starts adding up the minute you take out that advance. Make a payment to your credit card before you go, ideally to put the card into a credit balance. Then you don't get any interest charged when the balance is below zero. That goes for purchases as well as cash advances. Using cash advances on your credit card might be a good backup to your ATM card. See Number 3. You can also get a cash advance on on the credit card, common ones like Visa and MasterCard inside a bank though you may have to show your passport as ID.
3. Have a backup. Once in awhile an ATM won't accept your card. In that case i just used my credit card for a cash advance. Make sure you have a backup, either 2 atm cards for 2 different banks, or an atm and a credit card that you can get an advance from. If you feel more comfortable having those travelers cheques as a backup, go ahead. If you get them in local currency, chances are you *may* be able to use them in a shop as local currency instead of going to the bank to get cash.
I usually always buy some currency before I go away, to have something to start off with. You always need some cash for taxis, or the subway/metro/underground when you arrive. Good to have for cafes or corner shops as well.