I was pointed to this site by a fried - apparently the prices are good, and they price match other UK based sites:

Disney tickets, plus Flextickets, Seaworld, Discovery Cove, Universal Studios, Dinner Shows and other Orlando Attractions

Does anyone know if the prices on there are good, or is it better to buy tickets once you arrive? I've heard some people say that there are always special offers / vouchers in papers once you arrive that can get you a good deal.