Just to let fellow Mumsnetters know, I just booked travel insurance with Greenbee (0845 610 0305) as we're off to France for 2 weeks in August, and they only charged me an extra £3 to cover DS's asthma and milk/egg/nut allergies, which I think is a bargain! (Plus they covered my panic disorder, which most companies won't touch). Nut allergies are covered as standard too, you don't have to declare it as a medical condition.
Hope this helps - now all I need to do is brush up on my French to read all those food labels...