Downlighters

[riveralt]

We've got someone in to do the installation for us - I wouldn't go near electrics as, you have pointed out, I know nothing about them!

Evidently.
The person who is doing the installation should know the answer to these questions - why are you asking here? Does this mean you don't trust them?
Given the questions you have raised, the photographs you have shown the whole job appears to be a botch from beginning to end - Cowboys!!

There are no "new" electrics - simply a re-routing of 2 electric sockets.

And this is around a number of electric cables (the 2 sockets mentioned above and a number of other cables that come from the fuse box).

So the cables from the consumer unit also now run through this insulation
PLEASE - PLEASE - PLEASE get someone in who knows what they are doing - you are putting yourself and others in real danger.

Is this rockwool different from thermal insulation in that it doesn't create the need for the cables to be de-rated, or does it in fact require a higher de-rating of the cables given its density?

Where did you read that - if it meets current standards - which it does then you cannot put run standard sized cables through it without seriously endangering your life.