Highway Code Rule 267

Do not overtake unless you are sure it is safe and legal to do so. Overtake only on the right. You should

check your mirrors
take time to judge the speeds correctly
make sure that the lane you will be joining is sufficiently clear ahead and behind
take a quick sideways glance into the blind spot area to verify the position of a vehicle that may have disappeared from your view in the mirror
remember that traffic may be coming up behind you very quickly. Check all your mirrors carefully. Look out for motorcyclists. When it is safe to do so, signal in plenty of time, then move out
ensure you do not cut in on the vehicle you have overtaken
be especially careful at night and in poor visibility when it is harder to judge speed and distance.

Section: Motorways (rules 253 to 273)
Subsection: Overtaking (rules 267 to 269)