i am thinking of insulating between rafters

Hi also to maximise your heat retention Make sure all joints and gaps in the insulation are covered with foil tape

The insulation will be sufficient and a 50mm gap will be fine too

the air gap of a 50mm is enough, as minimum required, sometimes the rafters are not big enough for insulation and air gap, so you may need to add an extra timber underside the rafters so you can use a thicker insulation. also all gaps and joists need to be taped with aluminium foil tape. and regarding to your question, yes, ventilation is very important so i will recommend to use vented sofit board and vented tiles.

I would go more towards super quilt then solid insulation nowadays. Most of our projects are done with it better airflow easier to install.

If it just for storage. Look for a product called “ super foil”. Easier to use with a lot less mess and faster to install. The main benefit is it’s “ breathable “ Which is equally if not more important than insulation benifit.