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Highway code
Rule 133
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.If there is someone along side you in your blind spot it isn't safe so you shouldn't indicate.
jellybaby-
I'd argue that flashing a little yellow light to tell the person beside you that you intend to move into the space they're otherwise occupying is a rather necessary action and not unsafe.
Again, if you're travelling along and someone's in your blind spot travelling the same speed, what do you do if you're approaching a car in your lane that's moving slower than you, brake? The car behind you has no idea what you're doing. You plan ahead, indicate your intention, then slow if you have to and move over when clear etc.The highway code is great and all but I see people daily lurched forward over their steering wheel trying to check behind them in their mirrors before turning with no indication at all. By the time it's clear and they indicate they're already at the junction so you get 2 flashes of amber and a swing to the left
PhilDAS
Indicating is to indicate your intention, you should indicate whether there's someone in your blind spot or not. Otherwise you wait or slow down until your blind spot is clear or something and nobody around you has any idea what you're doing.