As I understand it the Single European Patent requires a role for the European Court of Justice as a legal backstop.
At one point Theresa May has said that wants no role for the ECJ at all, anywhere, post Brexit. This was when people still thought her opinions mattered in the grand scheme of things. Since then quite a bit has changed and the UK is now very likely to retain a number of links with programmes under EU law (Horizon 2020 for instance) and thus with the ECJ as final arbiter. The UPC is most likely going to end up being one of those things where the UK will decide to stay engaged with post-Brexit. The problem is that until that is 100% guaranteed (and it may take a year until that is clear) it is not easy to put a signature underneath approval for a next phase of the project.