Yes, you are required to turn on the indicators before changing lanes on a motorway.
The indicators should be turned on well in advance (but not so early that your intentions may be misinterpreted) before you:
Do not forget to turn off the car's indicators after completing the manoeuvre.
Before turning, moving your car sideways or reversing, you must always check your rear-view mirror and side mirrors as well as your vehicle's blind spot by briefly glancing over your shoulder. The blind spot is the area not covered by any mirror.