You can do this online at
royalmail.com or you can collect a special form from the post office. Also send some cards to friends and family informing them of your big event. There are various resources aimed at helping you remember everyone who needs to be informed, check out:-
iammoving.com or else
slothmove.com which will inform all your utilities at once and loads of others saving time with this tedious task. So many people to inform, don’t forget your Doctor, Dentist, Optician & Vet. Also the Electoral Register, Vehicle log books, TV licence, Local Council (tax) and HMRC Inland Revenue.