I really like Rosemary & Co... but theyre bases in the
UK, and are much smaller sized company than W&N are.
You can buy two Rosemary and co series 33 kollinsky sables for the price of a single W&N s7. Although I cant deny that my series 7 IS a lil bit nicer... but not by enough to say its better value.
No idea about the P3 brushes.. but Im pretty sure theyre 'real' wether theyre synthetic or natrual hairs

hehehe
Some synthetic brushes arent all bad

theyve got their place.
Fyi a good brush such as a W&N series 7 (or just about any other quality kollinsky brush) of any size will hold a point far sharper and far longer than just about any other brush, so you can certainly achieve super fine details with a large sized brush.
The brush size is actually about how much paint you want to be able to load into the brush. As such using a tiny lil fine detail one is often self defeating as the paint volume is so small it dries in the bristles super fast.
A decent size 1 or 0 is quite likely a very good all rounder, while 2's or 4's make great basecoat and wash brushes for the added paint volume.
GW's brushes..
LOL is all I can say about that.
Priced as premium kollinksy, but perform like kiddy craft brush. No spring or point holding ability to be described.
Their large drybrush is alright - the big fat flat one cant pick much fault with that. But just one W&N s7 or equiv brush will beat everything
GW have in the rack for all but the specialised jobs like drybrush and stipple
edit;
As for synthetic vs natrual; Those who do not thin their paints, might have a hard time using a kollinksy sable. They are water colour brushes primarily, as such are meant to be used with very thin paint.
R&C offer synthetic/sable blends which perform quite well, but wont hold the point that a pure kollinsky can.
So choosing synthetic of natrual might depend on how you like to paint? from the pot? or do you use a pallete and thin paint out (its better that way anyway)