Is it worth applying for PhD in the UK without the requested undergraduate average?
The short answer is you should apply.

Generally speaking, all your previous degrees and grades count towards your application for a PhD. But if you are having strong marks in your Masters, you should definitely try to submit your provisional scores to give an indication of your upward trajectory. Many PhD applicants would be in your similar situation. In many UK institutions, a provisional acceptance may be offered to somebody in your situation where the profile is strong, conditional on the completion of the Masters at a certain level. In other words, the final acceptance will not be offered until all the grades come in.