What is the best way to insulate this loft?

Moved into a new to us property, built 1947. Issue is the place is bloody freezing, we have cavity wall insulation but the loft insulation is 30-50mm at best.

All the guides I’ve seen say to replace the old with new, but our rafters seem oddly spaced over a large part of the loft.

We also have quite a few wires which I have been told is either completely fine to lay over, or they need lifting and moving.

Any advice would be greatly appreciated.