Welcome to AVS's DIY Screen Forum!
Where in the UK are you? I've traveled there a bit, doing a few Screen projects, from Glasgow down to Dover, with a stint or two in Dublin as well.
So what paints are available to use over there?
Dulux Light & Space Ultimate White in Matte (Lumitec Technology) Tinted to a neutral Gray of N8.0 value
Spray using a Wagner Electric HVLP (W500)
The Dulux paint is like putty-thick, so you MUST thin it using at least 25% water to Paint volume (25/75). I'd get 3 UK quarts of the Dulux & Quart of Distilled Water. Have the Paint Technician add all of the Paint / Water into a 1 Gallon "Pot" (...that's what they call "Cans" in Ol' Blighty"
) and have them shake it all together. Then remove 1 Quart of the "pure White Paint / Water" and place into a Quart "Pot".
Now have them take the remaining 2 Quarts and mix in the required N8.0 Gray Tint.
You now "lightly" spray a coat of pure white onto the BOC. Let dry. Spay another Coar. Now using a soft Sanding Sponge, lightly "brish sand" the BOC surface to smooth out the inevitable "Fuzziness'" spraying intp BOC creates. After the 3rd "White" coat, and one last light "brushing", the surface will be primed, and ready to let the Tinted paint go in and create a silky smooth surface.
Finish Coat spray technique should be using the "Duster" method, where you apply the paint from a distance of 14" away (35.5 cm) and move across the surface at 1 meter per second. You overlap each preceding row by 60%.
You should wind up applying a minimum of 4 of such quick, light coats. You can lightly sand after the second Finish Coat, but do not sand the last coat.
Last tip. Make sure all sides around the screen have a level backing (Cardboard - Wood - Taunt Plastic) so there is no drop=off or recessed ledge. If there is, you'll have issue with getting paint to apply evenly around the perimeter of the Screen. It will be too light. You need a continuing surface to run off onto for at least 4" then you drop your 40% of Row height and return the other direction. Never stop in the middle....never start in the middle.
Do all that and it will turn out brilliantly.