People who are Readers will usually use the job title Reader.
Where I am Reader is a category within the Associate Professor band plus a couple of additional spinal points. Some people who are Associate Professors never bother to try for Reader as you don't have to have been a Reader before becoming a Professor and it is just a title rather than a promotion in terms of money or spinal points (except the two additional spinal points at the top), so they just progress from Associate Professor to Professor (or when we had the old titles Senior Lecturer to Professor).
The way spinal points generally work is that you automatically progress up one spinal point within your grade each year until you reach the top of the spinal points covered by your grade, for example the spinal points in the Lecturer grade. If you then want to progress further, e.g., to the Senior Lecturer grade, you have to apply for promotion and meet the criteria to be promoted, otherwise you just remain at the top of your existing grade indefinitely. You can apply for promotion at any point, you don't have to wait to reach the top of the spinal points for your grade to apply.
If you start a job at a University, the appointments panel will make an assessment of where you should be placed on the spinal points for the job grade you applied for. Their assessment is based on your experience and achievements in relation to the job descriptors for the role and takes into account (but does not guarantee to match or exceed) where you were on the scale of your previous University if you worked for one.
The exact criteria for promotion to Professor will vary a bit between universities depending on what the University values, as well as whether you are in a teaching-focussed, research-focussed or teaching and research-focussed role and, to an extent, the general expectations in your subject area.
Broadly, where I am, a Professor is expected to have achieved recognised international eminence, evidenced mainly by a sustained track record of publication in the top journals in their field (some subjects use impact factors, some don't) and winning large and prestigious research grants as a PI. There are also expectations around both academic and administrative leadership so things like leading research groups, having big departmental administrative roles, serving on University or external committees, running conferences, editing journals, being consulted by government, appearing in the media, and so on can all be used to build a case for promotion. There are also expectations around teaching, both amount and quality.
Appointment as a Professor of Practice is rather different and there will be specific criteria depending on their subject, career and current involvement with academia, but the focus is on the esteem and importance of their career and how it relates to their involvement in academia (sometimes also their academic qualifications) rather than simply how long they have been in it.