Our build is more complex but we had the exact same issue that the ufh clips could not penetrate the below ground water proofing . Just used the ‘egg cup’ plastic ( forgive lack of terminology) . So ufh clipped inbetween those . Upper layer ( which is B and B ) did a membrane first , then pir - then sheet to protect pir from screed . So happily pierced that sheet with ufh clips .