Our ATM and credit cards in Australia are not chip and pin yet either, however I was in the UK in August, and had no problems with either my credit or debit card (I also used my debit card to withdraw cash from ATMs).

When completing a credit or debit card transaction in a store it's best to inform the checkout operator that you'll need to sign to authorise the transaction as you hand over your card, otherwise they'll assume you have a pin number, which is a different transaction type.

I have a feeling that I had to select a different account type to withdraw cash from an ATM to the one I normally use here in Aus (ie Savings instead of Credit). You may want to check with your bank before you travel.