I'm from the uk too. I am trying to get help to overcome my anorexia too. The edauk are very nice and they have a helpline which was my first call for help.
Your daughter is doing well just by recognising she needs to change her attitudes. That doesn't make it any easier to do this though. Unfortunately the ones we love are the ones who suffer right along with us when we have an eating disorder.
I am 23 and my husband bears the brunt of my anger when I don't want to do well. The thing with anorexia is that as much as you want to beat it there is always another part which actually wants to keep it.
I am a vegetarian and I know many anorexia sufferers find it hard to think of eating animal fats because they are more easily absorbed. Maybe you can gradually incorporate some vegetable fats though, just as a starting point. Not a lot, maybe you can have a 'safe' day of the week like on sundays when you are allowed to use a little oil in your cooking or she is 'allowed' to eat a mini yoghurt that isn't low fat or something. I must agree with Raindrop though, don't try and be sneaky and add stuff behind her back because it will only end up backfiring.
Other than that you need to ensure she keeps getting counseling and therapy and realise that these things don't change overnight. Her anorexia has worked hard to form these habits and breaking them is a long and painfully slow process. Good luck to your daughter and to yourself.