As you are in the UK, get on over to
www.hiroboy.com and buy some Tamiya Enamel X18. It might be old skool paint, but it's THE BEST for brush painting, and lays down almost brush stroke free.
I gave up on Acrylic X18. Even through airbrush it's iffy... I use TS29 for large surface Semi gloss, this is a Tamiya spray can, and is superb and I use Enamel X18 when I need to brush paint..
As for black under CF decals.. Get on over to Halfords, buy a can of spraycan PLASTIC grey primer, and a 500ml can of satin automotive black spray paint.. The plastic primer will protect the plastic from the hot automotive lacquer, and the satin black is PERFECT for a base for CF decals..
500ml for a fiver, much cheapr, and quicker... Tamiya Acrylic X18 is not all that good, you can make it work but is hard, even when airbrushed..
HTH
Edit - I do still use Acrylic X18 for window frames, but only airbrushed.. But thats about it after the last time I used the same paint on seats and it went glossy in places..