I have a Thule roof mounted rack. The 591 or something. It fits the alloy roor bars geat. Fitted to a Laguna estate with roof rails. Mine came from the roofbox company too and has been great for a couple of years. It is a bit pricey though Approx 70quid per bike plus the bars but well worth it. Much better than having the bike haning on the back for people to crash into.
I have the Thule Aero bars (591 or 571's) on my Audi A6 Avant, mounted off the alloy roof bars.
I bought the Halfords Pro bike carriers as they were 2 for £80 and the Thule ones were £70 each as you said. I couldn't cough up the nearly £300 for a fully Thule system as much I would have liked to, £230 odd quid was enough!