It is correct that you are not entitled to single persons disability. As for how much the difference in rate is, i don't know, and as Terpe said also you lose your 25% single person's Council Tax reduction. It does seem so unfair when your wife is not being entitled to public funds. You need your wife to get some kind of work to help.

I'm afraid it looks like you will have to tighten your belt, to meet the oncoming fees.

I hope you can find some way of making up the difference by some sort of benefit or other.

Mght be worth visiting Citizens Advice to see if you are missing out on any benefits etc