Highway Code Rule 167

DO NOT overtake where you might come into conflict with other road users. For example

approaching or at a road junction on either side of the road
where the road narrows
when approaching a school crossing patrol
on the approach to crossing facilities
where a vehicle ahead is slowing to stop for a pedestrian that is crossing from a pedestrian island (see Rule 165)
between the kerb and a bus or tram when it is at a stop
where traffic is queuing at junctions or road works
when you would force another road user to swerve or slow down
at a level crossing
when a road user is indicating right, even if you believe the signal should have been cancelled. Do not take a risk; wait for the signal to be cancelled
stay behind if you are following a cyclist approaching a roundabout or junction, and you intend to turn left. Do not cut across cyclists going ahead, including those using cycle lanes and cycle tracks (see Rule H3)
stay behind if you are following a horse rider or horse drawn vehicle approaching a roundabout or junction, and you intend to turn left. Do not cut across a horse rider or horse drawn vehicle going ahead
when a tram is standing at a kerbside tram stop and there is no clearly marked passing lane for other traffic.

Section: Using the road (rules 159 to 203)
Subsection: Overtaking (rules 162 to 169)