Honestly pal I'd skip the H&R springs, especially if you're on old shocks as lowering on the standard shocks is a recipe for a shock replacement within a few thousand miles - my front right failed on my 2012 S3 after less than 12 months on H&R 25 mm springs and my car has only covered 34k miles - or a bouncy AF ride cos your shocks are not right for your springs which I also experienced.
Get this fitted instead:
Specialist in performance parts for the VW Audi Seat Skoda Group cars. Interest free finance available on all car tuning parts and we have our fully equipped specialist workshop in Manchester.
www.awesomegti.com
These are the 2wd version but there is cabrio and quattro versions for the same price for the A3:
www.awesomegti.com
They're budget coilovers, but made by KW and they don't skimp on the quality. I'm now running this kit on my S3 with a 30mm drop (it will go as low as 50 if you really want..) and the ride is amazing, firm at times yes but very smooth and not harsh at all and miles better than it ever was stock or on H&R springs. Awesome GTI will fit for £199 Inc vat including full balancing, tracking and alignment if you purchase from them. Wish I'd fitted these when I first got the car and never bothered with the lowering springs which cost me around £400 in total inc fitting and balancing and were a total waste of money. Ok you'll probs pay £300 or so more in total for this coilover kit in total but you won't end up spending more in the future, trust me it'll be way worth it, and you can always have the height adjusted whenever you want.