I would go with the Vallejo Model Air RAF paints on this. they are a better quality and rather than scale altered are chipset effect and so will give the exterior a much better and business like appearance.
I am glad that you are going with masks for the insignia and remember that the blue is actually Oxford Blue, not insignia blue from MRP.
Oxford Blue BS105 MRP-183
Spray the roundel white and then put the masking back in the circle ONCE THE WHITE IS CURED, taking off, painting the colours letting them cure and then replacing the mask before moving on.
It is a complex thing and takes longer than decals, but the result is more realistic.
Don't forget that the squadron codes are Sky. Vallejo is the best for that!
The aircraft camo was hard edged and done with masks in the factory and I recommend Top Notch Masks for that to replicate it.
https://topnotch-success.net/vinyl-masks/camouflage/raf/spitfire-ixc-masks/
Also, use the insignia yellow for the leading edge ID markings but don't put a white background, just airbrush onto the camo paint and it will come out just a slightly bit deeper than the roundel ID yellow on a white background which will help to keep the human eye moving around the model.
Paint it as the originals were painted and the difference from decals etc is very noticable and more realistic.