ThePants999 - sorry, you are wrong here and BucksFizz is correct.
Indicators are to be used as an indication of the move you are about to make. One which you are about to make ONLY after you have actioned MIRROR, SIGNAL, MIRROR.
Indicators should never be used to say:
"I want to move out, please somebody let me out"
"I'm about to move out right now, I haven't checked if there is a safe space to move into, but I'm going.. right now.. oh yes I am..."
"I've come right up the rear end of a car, this is my instant reaction is to signal, for some vague intention in the future of moving round them"
A million times no.
Indicators are there to let other drivers know your intentions so that they can react.
Indicators are nothing to do with vague intentions like this - otherwise how would any driver know when you are going to act on those intentions? React? How so? Do you mean take evasive action because you are about to endanger my life by moving into my safe stopping gap?
This is such a dangerous attitude, but one shared by so many poor drivers.
Your indicator should not be on at all unless you have checked your mirrors, you have identified a safe gap to move into which does not involve causing any other driver to act evasively to avoid crashing into you or endangering their life, then you indicate, then you do a final check of your mirrors again to check for anything that has changed, and if at that point there is not already existing a safe gap for you to move into, you cancel your indicator immediately and start all over again.
Sitting in the middle lane with your indicator on whilst a queue of overtaking vehicles passes you in the outside lane (and therefore there is no safe gap for you to enter), meaning every single one of them is unsure if you are about to suddenly push out into their safe space at any time, is one of the highest displays of motorway idiocy there is.