Can't guarantee they are the cheapest but I've used them several times with no problems. You may save £1-£2 elsewhere but is it worth bothering about given how you spend in total ??
Last time I checked they had free delivery and a special deal on the 4-day base ticket.
Another vote for ticketmania.com. I ordered mine on a Wednesday and had them the following wednesday, for a 4 day (+ 3 free days) base ticket cost us aroung £110. each,
Or www.park-tickets.com - their rates are based on the dollar exchange rate which was 1.93 yesterday. They will deliver to the UK or to your accomodation or you can pick up in Kissimmee. Quote referrer 1781 (no I don't get commission!!)
I have found a great website with great deals on tickets. For a 14 day consecutive unlimited access to WDW parks including the water parks- 163 pounds per adult. The web address is www.themeparkticketsdirect.com so check it out. Hope this helps.