When should one apply for a PhD in the UK?
Everyone doing a Master's is in the same boat. PhD applications aren't usually due until late January or early February, so that's a good four or five months into your Master's course. This gives you plenty of time to work on the applications and get to know your lecturers to secure a reference letter or two.
Plus, you may end up working on something completely different for your PhD than you did for your Master's so it doesn't really matter how far along in your Master's work you are at the time of applying for PhDs. Furthermore, admissions committees are fully aware of this situation.
Good luck with the applications.