Re: RYA Certificate paths
You need a logbook. There is a section in the RYA logbook to record miles you have done previously as well. (Self certified)
Your day skipper course is usually two weeks, one of theory, and one practical. The same for the coastal level.
After that, once you get to the exam stage, it will be obvious to the examiner that you are competent, but remember you will also get theory questions to prove your knowledge.
I would recommend going on a YM pre-exam course to fully prepare.
All the details are in the G15 RYA logbook, or on the RYA website.
Hope this helps.