The new Allergy UK magazine has an article about this. It says that Wagamamas, the Japanese chain, will cater for allergies. It says that there's a page on their website which tells you what you can eat from their menu depending on your allergy or intolerance. That's at www.wagamama.com. If you haven't been there it's fantastic - good food and cheap. The only thing is that it's very communal. You sit on long benches that go across the restaurant, so not really the place for a cosy chat.
It also talks about "El Piano" in York, but it doesn't sound like the kind of place that you'd take kids. Am also sure that I just read something about the chefs and staff at Centre Parks all knowing about allergies and being very helpful.
We normally just end up giving ds chips when we're out as it's the only thing we can be 100% sure of.