I think your best bet is to identify potential supervisors at one or all of the universities you are interested in and make some informal inquiries. You will need to decide if you have your own rough idea for a topic worked out (if you have no idea at all they will probably see you as a bit of a timewaster) or whether you would prefer to do a phd which is already defined (i.e. it's part of a bigger project, or involves an industrial/private sector partner, or has been devised by a research council). If you see a pre-defined phd topic it should specify the entry requirements.
As far as masters degrees are concerned, there is a huge variation between courses and universities. If you want to apply for ESRC funding you will need an ESRC-approved masters degree (I believe it is possible to get your work experience accredited but no one I know has been able to do this and has been forced to do a masters degree before taking up their funding). If you get in touch with potential supervisors you should also ask about funding possibilities because some departments fund scholarships each year and there are also a small number of other opportunities e.g. learned societies, government-funded scholarships etc.
If you can pay your own tuition fees they'll be biting your hand off to accept you.
As an aside, a friend of mine is doing a marketing phd funded by the Food Standards Agency, she doesn't have a masters degree and worked for an investment bank for 2 years between bachelors degree and phd.
Good luck!