UK universities do assess PhD applicants on a case-by-case basis
Actually, UK universities do assess PhD applicants on a case-by-case basis. Broadly speaking, the criteria for admission are:
- is the applicant's research proposal coherent and will it advance scholarship?
- does the applicant possess the necessary skill, motivation, and credentials to conduct the research proposed?
- does the university have a member of academic staff who is capable of supervising the applicant's research and who is available and willing to do so?
(NB: getting admitted to a PhD programme in the UK is relatively straightforward, but getting a funded PhD place is significantly harder)
In general, the minimum "credentials" expected of an applicant by a UK university would be:
- an upper-second-class or first-class undergraduate (Bachelor's) degree in a relevant (but not necessarily the same) discipline; and
- (where the applicant's previous university studies were not in English) evidence that the applicant is highly proficient in the use and comprehension of English.
Expectations regarding a Master's degree
Nowadays, a relevant Master's degree would also be expected (but probably not essential), since it is regarded as good preparation for undertaking original research. For that purpose, NARIC equivalence is not very important here -- a UK university is more interested in the details of what you did in that Master's degree. Ideally, you will have written an original dissertation or project involving serious original research in a relevant discipline (the dissertation need not be particularly long -- the quality and potential are what count, and a potential supervisor may well ask to see only extracts from it), demonstrated specialist knowledge relevant to your PhD research proposal, and manifested some appreciation of some of the research methodologies you propose to deploy in your PhD.
So, if you feel that your existing Master's degree can demonstrate research preparedness and a suitable specialist knowledge for your research proposal, it may well be sufficient. You should be prepared to discuss what you learned and discovered from that Master's degree in detail, and show extracts from the work you produced in fulfilment of the requirements for that degree (such as a dissertation).