Just to concentrate on the refs only (there is a whole parallel pathway system for assessors and ref coaches).
The starting point is ELRA (or NFC as it was). Once passed you should ref mini/midi or U13 and a lot of coaching match scenarios. Some ex-players may be able to start straight off as an adult match referee, but it isn't recommended. Run as TJ a few times, ref coaching sessions at local club (adult and junior), and get confidence. Some stay as Club Refs, and the new ClubRefereeCoordinator in each club and ClubRefereeDevelopmentOfficer(s) in each county is a major RFU initiative to support Club refs, offer training and raise the standards. Longer term I see certification beking brought in (sorry Dave !) mainly as an insurance requirement.
About 10-15% of ELRA attendees join the local county Society. In the Society we have had post ELRA refs who were just not ready to referee (confidence, awareness, and safety) so we need to get them ready. That may mean spending a season or part therof as a club ref.
If in a Society you should be able to decide how often you ref, geographical spread, etc as Greg and Phil describe. One match a month or a dozen - all are equally welcome. Your progress as a Society referee, with assessments / advice / mentor / coach and speed of grade promotion (to your natural potential, and often a couple of steps beyond !) will depend on your effort and commitment.
One commitment we do ask for is the Monthly Training meeting, which in many English Societies now follows the RFU CRDA modules as one key agenda item in the meeting. Also Referees are encouraged to do it for themselves, with a Personal Development Plan (PDP) if they are ambitious and want to get up the grades.
Once you get to Society Development Squad level there will be another monthly meeting, with higher grade refs running sessions on key parts of the Laws/Game, detailed discussion of match scenarios and usually a dedicated coach. We keep a register, and we enter all course elements for CRDA and other courses on the RugbyFirst records.
For a few after that comes Federation Squad (appointments at 6 and 7), squad meetings, special training, assessments/coaches, and for even fewer Group (all level 5 matches are Group Appointments now), where it is even more intense, there are weekly debriefs and goal setting. The Referee Pathway document is well out of date as Dixie says, and I complained like hell about it at the time, as it sets false expectations and is very misleading. Even in the "old days" a post NFC or ELRA 1 & 2 referee would not be appointed beyond level 11 adult or U16 in their first season, unless a real 'high flyer' and has proved themselves. A level match would take 3 seasons for even the quickest rising referee - the majority of Society Referees will only get to level 9 over a nunmber of seasons, which is the grade of a highly competent and good referee.
For about 50 or so referees across the whole country, after Group comes the National Panel of Referees, with a full time team coaching, supporting, assessing, etc. I will leave that to KML1 to explain further if necessary.