The question is whether the UK wants to stay in the ECJ's jurisdiction for patent disputes. If it decides to stay in it then British companies defending their patent would have the ECJ as their final arbiter (after having gone through the UPC of course).
That is a different situation with regards to the US Supreme Court as British companies are not in their jurisdiction. They would only have to deal with the US Supreme Court if their case involved an American patent, not if it involved their own (European) patent.
I don't understand what you mean with your comment about 'unrelated UK issues'. The ECJ only deals with EU law, it doesn't handle 'unrelated UK issues' and Brexit will not change that.