The 1% was just a rough figure based on 30+ years observation may be higher. What you have to remember is the difference between London and the regions and department structure. I’ve worked in departments with Regional offices of c250-350 staff with 1 G6 post in charge of the whole office, and possibly 1 or 2 G7 posts. With that ratio unless you are very exceptional or able to move to London it’s essentially deadman’s shoes and people can be in the same grade for years,
London is different, there will be differences between the two types of offices. Offices that deliver to the public e.g job centres, DVLA test centres which may have similar structures to regional offices but CS have the ability to move to other London locations and so have a far bigger pool of promotion opportunities.
The second are the Head Offices, which depending on the department can be combined with public delivery functions,. These typically have a lot flatter structure with many more G7, G6 and SCS posts. These also benefit from being in close proximity and so there are far more vacancies and opportunities. You also have a higher flow between public, voluntary and private sectors and opportunities like loans, secondments and TP.
If you only work in London you can get a false sense of what most Civil Servants experience. c75% of CS roles are outside London.
One reality you may not be aware of is the special treatment for those on grad/fast track schemes, the extra assistance and the behind scenes horse trading. I’ve known really capable people not get an interview/job because the SCS in charge of a scheme insists a job is ring-fenced for a grad so their success rate looks better. This makes it harder for those following a more traditional route to get promotion. Unfair, you bet, but why risk your own career as you climb the grades the pool gets smaller and peoples memories are long.
I’ve certainly seen grads with a plan to go back to their home town stymied because they rushed to G7 then G6 in London then find they can’t get a level G6 role in their home region as they are no competition for local long standing G7s.
i’m on my 6th department in over 30 years, I’ve worked in the regions and in London, I’ve worked on cross department roles working with peers in other departments to deliver cross government so,unions. I’ve also been involved in recruitment and promotion panels in a range of departments.