Highway Code Rule 151

In slow-moving traffic. You should

reduce the distance between you and the vehicle ahead to maintain traffic flow
never get so close to the vehicle in front that you cannot stop safely
leave enough space to be able to manoeuvre if the vehicle in front breaks down or an emergency vehicle needs to get past
not change lanes to the left to overtake
allow access into and from side roads, as blocking these will add to congestion
be aware of cyclists and motorcyclists who may be passing on either side.

Section: General rules, techniques and advice for all drivers and riders (rules 103 to 158)
Subsection: General advice (rules 144 to 158)