I know how you feel - my son suffers from eczema and was diagnosed with allergies to dairy, egg, nuts, seeds, wheat and tropical fruit when he was 15 months old. It's very hard! We weren't given any advice at all about diet; the hospital gave us the results and then sent us away.
Anyway, I love the book 'How to Cook For Food Allergies' by Linda Bryce-Gardyne, which has lots of recipes which can be adapted for various allergies and explains how ingredients can be substituted in cookery. There's also a good baking book called 'Allergy-Free Desserts' by Elizabeth Gordon (it's American though and uses cup measurements). I've made quite a few tasty and successful cakes & cookies from there.
There's a vegan website called Parsley Soup which has lots of good recipes, some involve soya but can be adapted, and I've made some of the cakes by substituting wheat-free flour.
Beware the 'free from' section of the supermarket! There's nothing much there that doesn't contain egg. We recently started getting some food on prescription; the best make is Glutafin as the range of wheat-free is bigger. Gluten free doesn't equal wheat free! We get biscuits, pasta, crispbread, and sometimes rolls but they aren't too nice :(
Btw, some dairy-allergic people also react to beef and the by-products like gelatin. Worth watching out for.
For quick snacks, my son loves Doves Farm apple flapjacks (Sainsburys/Waitrose), Orgran Outback Animals biscuits and the Organix Goodies oaty bars/carrot stick crisps from the baby aisle.
Check out the Goodness Direct website for one-stop allergy shopping - great range of flours, bread mixes etc for whatever combination of allergies.
Hope that helps!