According to the highway code fitting a bell is only "recommended"A bell is a legal requirement when the bike is sold new.
Rule 63
Sharing space with pedestrians, horse riders and horse drawn vehicles. When riding in places where sharing with pedestrians, horse riders or horse drawn vehicles is permitted, take care when passing pedestrians and horse riders, especially children, older adults or disabled people. Slow down when necessary and let them know you are there; for example, by ringing your bell (it is recommended that a bell is fitted to your bike), or by calling out politely.Rule 66 also recommends that a bell is fitted.