Can you use your us debit card in London?

YES if the below conditions are met:

• You have sufficient balance in your US bank account
• The merchant/ATM in London accepts the type of card that you have (Ex: VISA or Master Card etc)

  The UK banking system requires cardholders to have a PIN (Personal Identification Number), so you may have difficulty using an ATM if you don't have a PIN. Why not check with your own bank before leaving the US?