In answer to your questions
1) He doesn't. But as with US NEXRAD data, he can at the very least be aware of the density of precipitation within a particular area.
2) A colour-coded tint would be displayed, reflecting the precipitation density (not type)
3) Yes. But airborne weather radar is much more subject to masking than a surface based mosaic.
4) This is nothing new. Rather than reinvent the wheel, might NATS not take a leaf out of the book of countries such as the US that already display this data for controllers.
Were a NEXRAD-like network to exist in the UK, the next step would be to make it available to aircraft via datalink - just like they do in the US.
It is amazing what could be done for both controllers and pilots if systems didn't have to be reinvented from the ground up in the UK.
2D