I have my booking with them for our trip in August. I have booked my WDW accommodation with DU, staying at the All Stars and the Animal Kingdom Lodge. They don't offer any discounted rates other than those offered by Disney itself. What DU have done for me is to make sure that any discounts that I am entitled to (Disney Club, annual passholder, discount code, etc) are applied to my reservation. I probably would have been able to get these discounts myself, but it would have meant constant checking of the internet to see what discounts were available, plus lots of trans-atlantic phone calls to WDW to repeatedly change my reservation. DU have done all of this for me, and my only involvement has been a few emails to my agent (Carole) at DU.
DU will also make PS reservations, book tours, offer advice, prepare an itinery, etc, etc. I don't know if they will book flights from the UK to Orlando, but I'd guess not. All of your reservation details are available on DU's website once they have allocated you a password, and they offer a secure server for paying by credit card.
Overall I think it is a good service, which aims to add value rather than provide discounts on price.
Regards
Rob