We used undercovertourist last year, found them very good. Not sure if they're the absolute cheapest though. We're buying our Disney tix in a package with DDP so it's only the Orlando flextickets, KSC and maybe a boat tour that we'll prebook (may just sort out KSC and the boat trip when we're there)
Last week I purchased 2006 WDW and Universal tickets through Attraction Tickets Direct in London. Real tickets not vouchers were in my hand seven days from the order. Prices were good as well.
i have done lots of looking lately flexi 5 park should cost around £125 -£130 and a ultimate disney ticket around £155 for adults kids prices vary and although your adult tickets can be cheaper the kids can be more,some offer a discount for 6 or more in a group,attraction - tickets seem very good