If you need to change lane, first use your mirrors and if necessary take a quick sideways glance to make sure you will not force another road user to change course or speed. When it is safe to do so, signal to indicate your intentions to other road users and when clear, move over.
134. Multi-lane carriageways - Lane discipline
You should follow the signs and road markings and get into the lane as directed. In congested road conditions do not change lanes unnecessarily. Merging in turn is recommended but only if safe and appropriate when vehicles are travelling at a very low speed, e.g. when approaching road works or a road traffic incident. It is not recommended at high speed.
135. Multi-lane carriageways - Single carriageway
Where a single carriageway has three lanes and the road markings or signs do not give priority to traffic in either direction
use the middle lane only for overtaking or turning right. Remember, you have no more right to use the middle lane than a driver coming from the opposite direction