I've applied to be in the audience of QT before, and from what I recall I was asked to state which party I supported. All applications and the data provided would surely be made available for FOI requests, so because of this, it would be incredibly difficult for the BBC to deliberately fix the slanting of a QT audience. I think what's happening is that you're underestimating the sympathies of Tory supporters (or anyone else who attaches themselves to the right) to issues you that you consider to be left wing.
If there is actually a bias in the audience, it is probably down to there being a difficulty in actually finding enough right wing people who want to attend. Generally, as political interest increases, so too does left wing thinking. Students, trade unionists, educators etc. are just some of the groups whom I suspect are more likely to apply, and these groups tend to be left wing.