The main factor in deciding whether to signal or not is to ask yourself “Is there anyone around who can benefit from a signal?” when approaching a junction, a signal should always be given in case there is someone unseen who then appears that needs to know what you are about to do, otherwise use your own judgement.
The whole point of a signal is to help other road users so be careful about the timing of signals. If you signal one second before hitting the brakes, others won’t have time to respond and plan their own actions, but if you signal a long time before taking any action, no-one will know what you are doing. Similarly, don’t signal to stop before a junction if you are stopping after it
Most of the time, the signal will cancel itself, but there are occasions when there is not enough opposite lock to trigger the cancellation e.g. leaving a roundabout. It’s always worth checking that it has been switched off.
It’s very easy to signal only if you see someone behind as most signals are applied immediately after looking in the mirrors. However, we need to signal to any road user who may be helped by it so don’t forget to consider those ahead and to the side of you.