Senior Registrars (SRs) were medical (or dental) practitioners who were undertaking, or had completed, several years of higher level training in a hospital specialty or Public Health, but had not yet gained a position as consultant (either by choice or because the competition was too stiff), thus differentiating them from the modern day Specialist registrars who are still completing training.
Some tried to obtain a higher degree: in earlier days normally an MD or ChM (or the local variant), but in later times a few aimed at a PhD (which involved more formal supervision) instead.
There was also a side pathway for academics with an international reputation (shown by publications and presentations abroad) to get onto the General Medical Council's Specialist Register (and thus become eligible for an NHS consultant appointment) on the strength of their research, without having the clinical experience normally required for any particular specialty.
A doctor may have a reasonable degree of certainty that they could progress to consultant status upon becoming a senior registrar, typically after the completion of at least four years in the role.
[1] Some people changed speciality at this level (e.g. from cardiology to geriatrics), switched to general practice, or went abroad, though a certain amount of retraining might be required.