Never heard of "Ranbow Country".
Buzz Rickson, the "McCoy's" don't tailor their jackets. They come in set sizes.
And their prices are at the top end of the spectrum....with several other manufacturers offering as good/better product.
Aero Leather, in your neck of the woods, custom fits/makes their jackets. They offer different leathers, with some different color availability...but not a lot.
Eastman Leather (again, in your neck of the woods), has a "Slim" A2 they just came out with, which it looks like you already investigaed;
They don't offer custom sizing (or, I should say they do to some extent, but they will not accept any returns on these items), and do not offer much in the way of different leathers/colors.
Goodwear Leather (John Chapman) is one of the most highly respected reproduction A2 makers around today.
He offers several A2 contract designs based on the original WW2 designs, different leathers based on what was originally used. Custom made. I don't know how willing John would be to vary away from the "standard WW2" specifications however, and the wait for a jacket is around 8 months.
In my experience, not many "authentic" A2 manufacturers are willing to wander away from the "classic" A2 design/leathers.
I suspect you may need to look elsewhere for a custom leather jacket maker, such as Johnson's Leather in San Francisco. If you were local to that area, I'd say drop in and ask them what they can do...get a fitting. Not being local, I'd have some concerns about trying to get it done from afar.
There must be somebody in London who does "custom leather" work.