I'm going on that trip to Slovenia with Ky, and I can't remember how many online booking and agents both of us have checked over the last month or so, and the price never got better. Sample what period does that flight sale affect? Strictly the summer months, or something into the fall?
I know with us as Ky said our dates were quite limited in that we had a 2 day window going there and coming back and couldn't really take anything but those days. Also we're going at the start of May so waiting wasn't really an option given the inflexibility of our depart/return dates.
Also as Ky said you if you can get a good deal to London you can fly out most anywhere on a discount flight. This site gives a list of low cost carriers in Europe
http://wikitravel.org/en/Discount_airlines_in_Europe. Make sure to take a look some local carriers for where you might land as every country seems to have a few of them and they may fly to where you plan to land from here.