I will repeat what I was taught. You only indictate if there is someone to appreciate the signal. That makes the driver think and look around rather than doing it automatically and therefore not concentrating. If you are in a dedicated right turn lane, stopped at traffic lights, do you indicate ? It is obvious where you are going to go there is no need to signal, it is the same when entering the motorway.Really? So why do most driving websites advise indicating?
Edit: The Highway Code does not mention signalling when joining a motorway.