The first question here is: do you have the academic qualifications to be accepted as a student for a research degree? Most universities require first or upper second-class honours in a relevant British undergraduate degree; some universities will accept lower seconds. If you already have a master's degree it is usually acceptable, whatever the class of your undergraduate degree.
These are the general requirements which will allow you to go through straightforwardly. If you do not have them it does not mean that you will not be accepted, only that a special case has to be made, which will require the strong backing of your potential supervisor. For example, if you do not have a British degree, the university will have to satisfy itself that your overseas degree is of a standard equivalent to a British one. Or you may have a non-degree professional qualification plus considerable practical experience, on which a special case could be made for your acceptance.
In general we would say that you should not be immediately deterred if you do not have the typical formal qualifications for acceptance. Always explore with potential supervisors whether a special case can be made. It may be, for example, that you could be accepted subject to doing certain extra study, or passing a qualifying examination. Remember too that if one institution rejects you, it does not mean that all will. However, if you have had several rejections it may not be wise to pursue registration. You may need to review your likelihood of success and come to a more realistic estimate of your abilities.
The second question is: what degree are you going to be registered for? If you are a beginner in research and do not already have an MPhil or an MRes (i.e., a master's degree awarded for research) you will, in the first place, be registered as a general research student or for an MPhil degree. You will often be required to take some taught courses before embarking on your thesis work. You may be required to complete successfully a one year taught programme leading to the award of the MRes degree. The decision on formal registration for the PhD is then taken after the first year of your research when there is some indication that the work is progressing satisfactorily. You and your supervisor(s) must, therefore, be in close contact to ensure that the case can be made for full PhD registration. At this stage a title for the thesis and the intended programme of research are presented.
The third question is concerned with the limits of the period allowed between registration and submission. For full-time students there will be a formal minimum time (three or four years) and a formal maximum (four or five years) after which registration will lapse and a special (and very persuasive) case will need to be made for reinstatement. Because of this maximum limit, if you are having to abandon your research work temporarily but intend to return to it, you should obtain a formal suspension of the period of study.
For part-time students the time limits are set roughly pro rata: four to five years minimum, seven to eight years maximum. Don't forget that if you are employed by your institution as, say, a research assistant, you may find that you can be counted as a full-time student even if you are working only part-time on your PhD. This fudge is allowed because the basic nature of the PhD is as a professional training, and research assistants get a great deal of this training as part of their jobs.
When registration has been completed you should be informed formally of: your supervisor(s); the topic or field of study for which you have been accepted; the minimum length of study time required before submission of your thesis.
Continuing registration in succeeding years is usually dependent on adequate progress being made each year, and a report to this effect has to be submitted by your supervisor.