Traditionally, the key choice in applying for a PhD has been about who you would like to supervise you. The 'fit' between you and your research and your supervisor is one of the most important elements in having a successful and enjoyable experience of doctoral work.
It may be that you already know a key person in your field, or there may be someone whose work you really responded to, from reading done while you were doing a Master's, for example, who seemed to you to be doing exciting work that chimed with your ideas and approaches to a topic. It is these academics who you could consider approaching about doing a PhD.
Can I approach academics I don't know?
It is perfectly fine to write to academics who you don’t know, but whose work you have read and enjoyed, and ask them if they would look at your research proposal and consider supervising you.
Do beware though, of sending exactly the same email to lots of people (especially if some of them are in the same school or department).
How should I approach potential supervisors?
Potential supervisors want to see that you have thought about who to approach and that you have a real knowledge of their work because it shows you are thinking like a potential researcher and are able to read, assess and evaluate work in your field.
Write a unique email to each person you approach, making sure that you address them appropriately, using their correct title and surname. If they sign off with just their first name in replying, it is fine then to use it in subsequent emails.
Are there differences between disciplines?
Expectations and processes can vary across disciplines.
Very often in STEMM subjects, for example, PhD places are offered as part of a collaborative project, and you may be attracted to the project initially rather than the person who may supervise you when you join.
If you have questions about how the initial stages work in your discipline, the best thing to do is to contact the admissions tutor for PhDs in the department you are interested in.
You can find the contact details of admissions tutors/PhD coordinators at Westminster on the specific research area page you're interested in.
Do I need to choose a specific supervisor?
You might be thinking about particular universities, rather than supervisors, which is fine. You may be thinking of a particular geographical area, or thinking about a university where you have already studied as an undergraduate or Master's student.
Your eventual supervisor will still be important, and you will need to see if someone in the university you are considering works in your field, or in a closely related field, and so could act as your supervisor. Again, if you are unsure, you could contact academics there to see if they could act as your supervisor or contact the relevant admissions tutor/PhD coordinator.
Making contact with people to ask them questions or ask their advice is completely fine, and is considered normal when people are thinking about doing a PhD.