Alex,
One would think the various sellers in the UK could supply this? Try the
MilWeb Military Paint, Canvas and Consumables section, there are two advertisers there currently.
Another option could be this supplier in Germany
militaerlacke.de, they are focussing on military vehicle restorers (at least more than the generic paint suppliers do). For example, they claim they mix their paint in matt, semi-matt or gloss, rather than matting down a gloss base paint. Prices are not too bad either. They carry post-war British colours, maybe they are willing to add the wartime series to their range?
Let us know what you end up doing, eh?
HTH,
Hanno