I dont know about winter wheat, but if you can use rye grass ..... why not? However, when I was researching this up a week or two back, I did find several comments about rye grass that it was hard work to dig in with a spade. Easy for farmers with a plough, but not so good for allotmenteers. (no doubt someone will come along and say it is a breeze.
) I would think the same would apply to wheat.
I think you will find you are a bit late for mustard but the best site for info I found was greenmanure
and the best for prices was moles
Their service was good too.
Oh, and if you decide to go for winter tares, people here recommend the best prices are from the pet shop where they are sold as bird seed.