if the roof timbers are 150mm deep, you can lay mineral wool to that depth and it will give you pretty good insulation, while also allowing you to part-board the loft (if you want to) for storage or occasional walking.
The modern standard is 250mm insulation, which is better, but due to diminishing returns on increased thickness, not so much better that it is worth losing the ability to wak around in the loft.
If the timbers are only 100mm deep, I would be thinking about increasing the depth. There are a few ways you can do this that we can explore.
Assuming you are not short enough to want to walk about in the edges of the loft, you can certainly fill to timbers depth, and lay an additional layer over the top, using a roll about 1200mm wide.
You must not block the eaves which provide vital ventilation to prevent damp. Preferably you should be able to peer down and see daylight on both sides of the loft. If not you can pull back the quilt a bit, or trim it with giant scissors. It is OK to rest on the inner leaf of bricks and the cavity.
Brown mineral wool is likely to be the more modern sort, much better than old yellow fibreglass that sheds irritant dust and fibres. If you buy any more, look for rolls treated with "Ecose" to prevent it. It is made by Knauf but also sold as an own-brand. Ecose will be marked on the roll wrapper.
The felting or membrane will keep your loft reasonably clean. Felt is thick and stiff, made with tar. membrane is like a thin soft cloth fabric, it is a breathing material like gore-tex, so is better.
The W shape holds the roof in shape and must not be cut or removed. It shows that the roof structure has been built to a calculated strength sufficient to hold the weight of the tiles, snow, and the plaster ceiling, with litte extra, and insufficient for grand pianos, filing cabinets, or parties.
Scottish roofs are generally different from English roofs (better).