For those of you who have stocks of UK money left over from previous holidays, the new one pound coin is officially being introduced on Tuesday 28th March. You then have until Sunday 15th October to use up any old-style pound coins you have, after which you will only be able to exchange them by paying them into a UK bank account.
The new coin is being introduced because the old ones are heavily counterfeited - it will be bimetallic like the current two pound coin, and have a twelve-sided shape similar to the old pre-decimalisation three pence coin.