Other key constraints are the availability of matches at levels above 6, and overall the human resources (all volunteers these days except four or five paid staff at Elite and top of PNR) that we have to act as Referee Coaches, Match Observers, Assessors, Advisers, Mentors, etc and the people like me who manage the process and people involved as Grading Chairman / SADO-FADO-GADO.
I agree with Keith's comments and we as a Society, Federation and Group develop and promote based on evidence, commitment and achievement. The constraint is of course who gets the opportunity to show those facets, and that is restricted by the rugby pyramid of matches at each level declining. It is a competitive environment, and development referees are selected for advancement in the same way as players are selected for bigger clubs, representatives sides etc - all on evidence.
As I have said elsewhere level 6 is the highest level any Society appoints to, in some cases (South East Federation) it may even be Level 8 or 9. At L5 the four Groups appoint and that level is used as the testing ground for potential Panel Officials and as a level with nationally consistent protocols, standards, and process - part driven by the clubs coaches and DoRs.
The only referees selected for the Group squads (usually just 10-12 referees per Group, so under 50 across England = very competitive selection process) are those with demonstrated ability to be a National Panel Referee (as outlined by Keith above) and able to commit to being a member of PNR for a number of years to give a decent return on the financial and human resources put into their development process.
So I do not see it as a glass ceiling, but a mainly transparent evidence based selection process that rewards evidence of ability, performance, commitment, and potential,. As Keith mentioned we have two referees who have already made it to Group in 2-3 seasons, with a third close behind. All three are very fit, have good level playing experience, referee two/three times per week, have high levels of commitment and dedication, analyse and develop their own refereeing alongside their coaches/observers, and spend more time on their rugby than players at their level (some of whom are semi-pros).