I got the same speech from a number of rack makers trying to get a cycle rack for my Honda Civic Type R, which has a far bigger spoiler than the GTI. In the end a chap at a Seat dealer sent me to a local bike shop in Congleton and I spoke to them and purchased a random fit anything rack.
It attaches with two straps between the roof and rear hatch, two down on the side between hatch and rear quarter panel and two straps under the bumper. It had to large plastic bits on the metal that touch the glass and lower tailgate. I have since had that on the Golf as well and had no problems with it at all. It mounts the cycles quite high with the wheels being level with the glass and everything else being above the car. Not the best for fuel consumption I know but it fits and it works. Never had any problems with straps resting or putting weight on the spoilers.
I have seen similar in Halfords with the high level mounting.