Welcome to AVS DIY Screens! How's everything in Ol' Blighty?
Are you in / close to London? If so I can steer you to a few locations where you can see some humongous sized DIY Screens in operation in high ambient light conditions. Pubs / Football / Rugby Clubhouses and the like. Tell any of 'em to pull you a pint and send me the tab. They'll love that....
You can use Dulux Light & Space "Matte" (Lumitech) for your "white' and there are a few Water based Clear Satin Polyurethane "Floor varnishes" that will work. (I've been over there and that's what I used for any / all screens that were made...)
The Rustoleum Metallic Accents Pearl....you'll have to order that online. If you have any issues, PM me and acquire it for you and get it to you via USPS Priority International Flat Rate.
As for Substrate....consider a Acrylic Plastic Mirror. (3mm thick)Strangely....they seem to be far more abundant in large sizes and reasonably priced in the UK these days than here in the US. Using a Mirror allows you to use a much darker shade of Gray without overt attenuation. Light Fusion, don'tcha know. Most all the biggest screens in high ambient light situations I previously referred to are such.
Beyond that, Smooth, Expanded PVC Foam board is the next best thing to consider. Also known as "Sign Board", it's made to apply paint directly to. It will more than suffice.
Lastly, be advised that all the most effective DIY Screen paint applications require...nay, demand that they be sprayed. Wagner makes/sells a version of the same Electric HVLP Gun we use (Control Spray) but over there it's called the W500 or something close. Just do a search for "Wagner Electric HVLP' on yur UK-based Search Engine and it'll pop up.
Throughout the process, post up any / all questions or concerns before they become "past tense" ones. We are here to help, and although we can and will also provide damage control...it's a lot better to be pro-active as far as getting all the info you need before issues arise.
Now go out and quaff a Black & Tan for me, will ya?