This is a �should do� in the Highway Code. That is to say you cannot be prosecuted for failure to use your indicators. However, it may be held against you if, say, you were involved in an accident which may have been caused by such failure.
Additionally, it may help determine whether a prosecution for other matters, such as careless driving, is brought. If you pull out in front of someone without signalling the police may be more likely to pursue a charge of careless driving than if you had pulled out with an indication.
So, in short, you don�t have to, but you ought to.