In short there is no best time to book for Aug flights. You will struggle to buy a seat under £500.
Are you wanting a standard 2 week holiday? If you are then try TCD, They have just this week started there summer sale for this year, so I guess around mid June next year for them.
Do you have Virgin airmiles? If you do they released Miles & money seats on Xmas day 06 for August this year, I booked them at £533 each. Full price for Virgin this August has been around £800 each. but they might release at anytime after they come on sale 11 months out.
Can you get to Glasgow? Flyglobespan do good prices from there but they are a lowcost airline and have the problems that go with that like lack of back up planes etc.
You can book the Charter flights now but they are usually expensive.
Scheduled flights are not released until 11 months from your return date, So for Return end August you need to look around from end Sept. You might pick up an indirect flight and save on the direct routes, check out
www.moneysupermarket.com for a good search engine.
Other option is check every day 3 or 4 times a day, Companies make mistakes, BA released flights for this August around £300 and Expedia had flight/hotel packages for 3 weeks at £500 ish both were errors but were honoured. People post on here usually if they find a bargain like that.
Good luck its not easy in August.