See Highway Code Rule 105 - following driver should leave enough space to pull up safely if vehicle ahead suddenly slows down or stops.

Duty of following driver is, so far as reasonably possible, to take up such a position and to drive in such a fashion, as will enable him/her to deal successfully with all traffic emergencies reasonably to be anticipated. Fulfilment of this duty is a question of fact in each case (Brown & Lynn-v-Western Scottish Traction (1945)).

Generally, the vehicle colliding with the one in front is 100% liable BUT where vehicle in front brakes suddenly without reasonable excuse, contributory negligence will apply around 25%. However, see Thompson-v-Spedding (1973) – 50/50 – surprisingly there was one vehicle between Claimant and Defendant who braked suddenly and the middle vehicle pulled up in time.