Best insulation for pitched roof loft conversion

[bice]

I have a half-completed loft conversion in a Victorian SW London house from previous owners. It is not a real loft conversion as the Victorians built the house with a top bedroom under the eaves, so completing the loft conversion will get me a raised platform of more space and a higher ceiling and sense of space.

My question concerns insulation. There is a new roof and the roof has its original timber sarking boards.

The previous owners stuffed fibreglass wool between the rafters against the sarking boards and made a half-hearted attempt to plaster board.

There is no evidence at all that this has caused condensation (or had much effect, as the attic is freezing and draughty in winter). In summer the attic is alarmingly hot as they put in a large Velux window.

I want to sort this insulation out.

I am assuming it would be best to leave a 50mm gap to the sarking boards? (There is a gap between the sarking boards and the tiles which are nailed to battens attached to the sarking boards.)

What would be the best product to carry out this insulation between the rafters: Xtratherm Rafterloc / Moy Isover Metac / Kingspan or just best quality fibreglass wool?

Here is an image of the loft.

Thanks