From the Highway Code
In fact moving from right to left lane a car does not HAVE to indicate (as told me by a traffic cop) as drivers should not be undertaking & should be ready to give way if they need to move over. It is generally considered a Good Idea(tm). I see no difference though between allowing a car room to move in if they are moving from a side lane or if they are overtaking unless you are a of a persuasion to not let anyone get infront of them for fear their manliness will be tarnished.
Personally I will always give another road user room to pull in...and I would prefer that they gave me a clue that they are going to do so by using their indicators...
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.