Hi David
Tho I'm not even British, I think the best way to get ww1 aftermarket decals is Hannants web store at
http://www.hannants.co.uk/
They have a very nice search engine and you'll find easily for example all 1/48 Fokker military aircraft decals. Hannants sells the products of most of the few manufacturers that do put out ww1 sheets; AeroMaster, Eagle Strike, Blue Rider, FCM...
Do not expect too much tho. There are some sheets for the most popular plane types, like Fokker Dr. I and D.VII, none for most of the ww1 aeroplane types...
Mikko