20mm equal plastic stop tap and a 20-15mm internal copper adapter
Eg
https://www.travisperkins.co.uk/mdpe-fittings/plasson-adaptor-for-copper-pipe-15-x-20-mm-7438015/p/845157
https://www.travisperkins.co.uk/mdpe-fittings/plasson-stop-tap-20-mm-3407cc0/p/856957
Easiest way is to file and then Emory cloth the copper or if your good enough use a grinder carefully
As the copper is thick wall you will be fine taking .25 off