Yes, you will find at the moment that ticket prices are a little higher than they were say end of last summer/just before Xmas when the dollar rate was better for us Brits!
We bought DC tickets about 3 weeks ago now, imo we bought just at the right time as the price went up £9 pp within a few days. I believe this was down to the $ rate as it dropped at the same time. While £9 isn't a lot of money, we were buying 6 tickets so it soon mounts up. We still need US tickets but I am holding off on those for now - no hurry to buy.
ATD definitely aren't the cheapest these days. I read an interesting thread on another board about how their prices tend to include a 'buffer' for their 'freebies' they give out

. They will apparently price match if you call them up, but honestly, I'd rather go with someone who gives a decent price in the first place!
We bought our tickets through Orlando
attraction tickets as they were the cheapest at the time, offered no CC fees and free signed for postage as well as real tickets. We had no problems at all with them.
Orlando Attractions and
Undercover Tourist or Travel Republic are mentioned a fair bit too. I think it's a case of just keep watching their prices - not the best thing to do in your spare time, but it's the way to keep prices manageable when you are doing things DIY!
One tip - of you can, pay on CC, so that if anything were to happen, you have back up of the CC - on anything over £100.
Just to add - you can pay by £10 deposit pp with OAT too, although we didn't.