I have floored the loft in my daughters house (for storage only). I considered the counter batten approach to add depth for insulation, but with the added weight etc.
My solution was to add 4x2 directly on top of the existing, screwed every 16”. While this adds the same weight as counter battening it also adds strength and rigidity. I drilled to a depth of approx 3” with a 3/4” bit, then used a 2 1/2” screw and cup washer to secure on top of the original. This practically doubles the joist depth and adds strength that counter battening wouldn’t. Job done.
I’ve done mine and my inlaws’ lofts with the Loftzone Storefloor kits, they’re expensive but I think allow you to be a lot less accurate with their placement as you end up with 100mm wide ‘beams’ to lay your boards on.
You miss every other leg out so you can always skip the odd high or low joist but if yours are old and all over the place you might be better off with 2x4s.
With 100mm roof trusses you need another 250-300 height on top to allow for 200mm extra insulation and a 50-100mm gap for airflow under the boards.
With the plain loft legs you need to get one exactly on every join where 4 boards meet which could be awkward I imagine.
I fitted the wooden folding type spring-counterbalanced ladder-hatches and they’re very good, solid feeling and easy to fit. (Come pre-assembled)
I got these: