I'm not in the UK at the moment but when I searched the aliexpress site which aphybrid quoted as £52.82 the site came up in German , and the price was £25.58
But even £25.58 seems expensive to me for 4 bolts and collars. .
I dont know the exact sizes of spacer and bolts but a quick search on fleabay I think I could could get suitable aluminium 'stand off' spacer collars ,(in a choice of heights )and suitable allen screws for about £15 from uk suppliers. possibly even in black anodized aluminium.
But has anyone actually done this modification on a mk4 ? . The adverts for the kit show an illustration indicating its only suitable when all 4 rail mountings are on a simple flat floor. I cant access my car at present but IIRC the rear two are not on an entirely flat floor and may be quite complex. It might be possible but personally I wouldnt want to spend significant cash without being sure . And sure it wouldnt invalidate my insurance. (or even fail the MOT when the time comes, although thats unlikely unless it looks a complete dogs dinner)