Can I use coinage from the Channel Islands or the Isle of Man?
United Kingdom Dependent Territories, for example, the Channel Islands, Isle of Man and Gibraltar have their own legislative and taxation systems and issue their own banknotes and coins. These, in common, with United Kingdom coins bear the portrait the Queen but they are only legal tender within the territory itself.


from here

http://www.24carat.co.uk/legaltenderframe.html