I have several questions for you in order for me to help:
What food are you feeding her now?
Is she allergic to house dust mites or is she allergic to mites found in dog foods (grain mites)?
What other treatments have you tried? What dose of steroids is she on now?
What breed is she and how old is she?
If my dog were allergic to grain mites, I would absolutely NOT feed any food with a grain in it. I took a quick look at the bakers complete dried food, and most seem to be made with cereals. It doesn't say what specifically, but corn or other grains would be my guess.
You should look into feeding your dog a raw diet, that does not contain grains. There are several available commercially in the US, but I don't know about the UK. However, you can make your dogs food fresh at home as well, it just requires a little bit more work. Raw diets will be free of grains and hence grain mites. This would mean no biscuits either as treats. Your dog will be much more comfortable without these allergens being fed to her daily. Steroids have serious long term side effects, and it is not good to keep her on them long term.
Here is some info for you:
http://www.holisticvet.co.uk/canine_nutrition.htm , the section called "The Rules" should help you with feeding your dog a raw diet.
Let me know if I can help further.