Will my Visa debit card and atm card work in the UK?

I have a debit card with a Visa logo. Here in the states they ask me if I want to use it as credit or debit, and i always use 'credit'. Will it work just as well in London?

My atm card (and Visa card) have the Cirrus & Star logos on the back. Are there Cirrus atm machines in London?
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