Hi Simpson. I had this problem too. DD has more or less outgrown her intolerances, but she couldn't have diary or soya. DS now can't have gluten, dairy or soya.
I used a breadmaker, but for days when I couldn't be doing with it all, I managed to find a few dairy and soya-free loaves. I think Duchy Originals ones are OK, as are some by Cronks. Beware a lot of the bread mixes, as they seem to contain soya as an emulsifier.
For rice milk custard, try using slightly more custard powder than the instructions on the tin specify (2 very heaped tablespoons is good) and slightly less rice milk. The rice milk is so sweet that you can use less sugar too. Also, don't be too surprised whrn the finished custard is a weird golden colour, instead of that pale yellow. Don't know why this is!
Organix make some dairy and soya-free treats. Their chewy bars are good, as are the animal biscuits and ABC biscuits, but stay off the moon biscuits as they have milk in them. Most sorbets don't contain dairy or soya (but do check). These make good ice cream substitutes. Also Booja Booja now do a DELICIOUS range of dairy and soya-free ice cream-type desserts, but you can't get them in supermarkets - try health shops or organic shops.
If you're looking for chocolate, the Lindt 70% stuff is good and dairy and soya-free. It melts nicely and you can cook with it. Kelloggs Coco Pops are dairy and soya-free. I know they are not hugely healthy, but it's not like our dcs can gorge themselves on an awful lot of crap, so I do make some allowances for them!
For margarine, the only one I have found is 'Pure' sunflower (the one in the yellow tub). It tastes fine on bread or toast and makes perfectly good cakes. Only things I have yet to get it to work for are chocolate crispie cakes and flapjacks - both crumble to nought.
I hope this helps! Good luck!