Kinnerton actually do a dairy free/nut free egg in a Bart Simpson box. It is a dark chocolate egg but only 50% cocoa solids so not quite as bitter as 'adult' dark chocolate. The egg comes with jellies so it is a proper kid's egg. Green and Black also do Plain and Maya Gold eggs which contain no milk ingredients.
All are available in larger Sainsbury stores, some health food shops or from www.soya2u.co.uk.
D&D chocolates also do dark chocolate or carob easter goodies and I believe they have a web site now.