The HSE says no live working, so following their rules one must disconnect the supply before removing the cover from the consumer unit, elsewhere, not in the consumer unit its self.
So the supplier should provide a means to disconnect the supply, however often they don't, and two wrongs do not make a right, so one could ask the supplier to disconnect while the electrician works on the CU but there is nothing to say this must be free of charge.
So often we break the rules, we remove the DNO fuse and fit an isolator, and then the CU, and we then say we have noticed that the seal has been broken on the fuse, would they like to replace it, and we both know rules have been broken, and we both turn blind eyes to it.
In the main the DNO is only worried about the seal if they think your stealing electric, but it is like the 30 MPH speed limit, one can get away with speeding many times, but then some one will get done for just 4 MPH over, it just depends on the guy who catches you.
I know the seal on my fuse is broken, and I have photos of it being broken over many visits from meter reader, only as that the way I record meter reading, and the fuse is often in the picture, clearly in my area no one is worried.