Whilst I'm sure there are some reputable places to buy tickets on I-Drive, many are not. As far as WDW tickets are concerned, if they're discounted there's usually a catch (either you have to sit through hours of time-share spiel, or they're resales which is strictly prohibited and you can't be sure what you're buying).
I would wait until you arrive at your first park to buy your tickets. Those available at WDW differ from those offered by many companies selling to the UK market. WDW has just completely overhauled its ticket options, so I would take a good look at the information available here on The DIS.