No you do not have to be a club member, and about 10-15% of our Society's referees are not.
You do ELRA1 and 2 and join the Society. We appoint (very carefully) to 6 matches to qualify for ELRA 3 and check for safety in the first match and do an assessment for grading at match 5 or 6. Some Society referees may never get beyond level 12 (our lowest) and are very happy to referee the 3rd or 4th XV, or perhaps Ladies 2nd XV rugby.
Many referees prefer to spend a season doing U13 / U14 / U15 etc matches and then join the Society to complete ELRA 3.
In some cases we might advise a new no-club member referee to join a club first and do these youth matches if they do not have a lot of playing experience.
The scale is very different in England to Spain and there is over 100 years historical legacy, traditions and infrastructure.
The Society makes match appointments, recruits and trains / develops / assesses referees, holds monthly meetings - increasingly we are also working with clubs and embryonic club sub-societies as well. The objective is to provide an appropriately qualified trained safe and compteent referee at every level of game for clubs in our area (county).
The Society works closely with the County Union and RFU Referee Department who provide lots of guidance and training material, full time employees to work with us and they organise the Elite, National Panel and Groups.
There are 45 or so Societies across England with 5,500 members. 12-15 referees are selected in each of the four regions (London & SE, South West, Midlands and North) as Level 5 group refs (with L5 coasches & assessors), some of these may be selected for the National Panel of 45 or so referees (again with Panel Coaches and Assessors) and finally 6-8 are the Elite Referees (Messrs Barnes, Pearson, Doyle, Small, etc) who do Premiership, Heineken and some of them are awarded IRB appointments (at Elite level again with dedicated coaches, assessors, and advisers).