It's quite unusual for an employer to keep a mental note of where your university is ranked, especially for specific courses, unless you're going into particular fields. This is largely because ranking doesn't tell them much about the course itself, especially at undergraduate level, and frankly I doubt they have the time to thoroughly investigate the content of every course in the country. What they would be interested in is what you did, the skills you obtained and so on, as well as your grade; this is largely done by you on the application form and in the interview (again, unless you're entering the handful of fields which really do examine university courses in some detail, or those which have industry partnerships with some universities). Some employers filter out applicants by UCAS points and in doing so limit applicants from a top band of universities, but it's incredibly unusual for ranking to be used as a deciding factor.
Relevant experience is generally valued above a degree. Once you've been doing the job for a few years, nobody will really care what degree you have unless you're using an old boy network of some kind. But the degree opens the door into the field, and it can be harder to obtain work experience in many fields without a degree. Degrees are also the door-opener into a company's grad scheme.
I think PhDs are done for a variety of reasons, chiefly career-oriented, but also for prestige (hello, I'm Dr...) and genuine interest in the subject.