With load on there at driving speed, there will be a LOT of rearward force (+ lift with a roof box). With only that friction mount on the inside, that will fly off easily

Those circular indentations are used by the Thule 4902 feet (there's Q7-specific fit kit) to hold the rack snug to the rails. The angle of the clamp also makes it difficult/near-impossible for it to be forced off.

Since you already have the crossbars, they may be transferrable to the 4902 feet/fit kit. Use that shop in the UK -- great service, decent price. (IIRC, www.roofrackshop.com)

ButchV