There’s a few ways to go about it and some of it may depend on if you want/need to comply with B Regs.
If you’re really keen on keeping the lath & plaster, you could install insulation between rafters to full depth, install your breather membrane on top, fix 38 x 38mm counter battens to the rafter and then install your battens and tiles/slates on top. However 75mm deep insulation wont get you anywhere close to the min requirements.
To increase insulation, remove lath & plaster (or potentially clad over it) with insulated plasterboard- thickness as much as you can really. Have a look at Kingspan K7 and K18 boards. You can download PDF literature from their website.
You don’t have to use counter battens but if you don’t you’ll have to leave a gap which reduces your insulation thickness