This same site has also recently been added to the 'pull down' menu where i beleive you do pay to be listed.
I find it too much of a conicidence that they start paying to be listed and then appear in the 'editorial' answer section.
Sometimes it pays to go to the source itself. On the Ask UK site if you click on the "i" icon you find Ask's explanation. Basically, it says that they are sites that the editors found "particularly useful" and that "advertising partners may sponsor some of these results." It also gives a link to Ask's editorial policy.
Ask advertising calls these "Answer Links." More info here: