Is loft insulation flammable

Loft Insulation fire ratings explained 

Below you can see the Classifications and descriptions of each fire rating. This will give a better understanding to each if you are not experienced in this area.

https://www.knaufinsulation.co.uk/media/news/a-beginners-guide-to-insulation-fire-safety

Can you lay insulation over electrical wires in the attic?

Yes loft insulation can be laid over wires, most attic insulation such as rockwool or fibreglass is naturally fireproof  but you should consult an electrician to make sure because too much insulation can affect the current carrying capacity of the wire. 

Loft insulation over downlights.

Downlights can be a minor fire risk if not installed correctly.

Downlighter covers eliminate the risk of fire starting as a result of contact between the downlights and the insulation or other objects stored in the attic e.g. Christmas decorations placed on downlights.

Also check How thick should loft insulation be? before applying new insulation in your loft space.