As others have answered, @Delectable, the ban on the creation of new grammars stems from the 1960s with the move to form comprehensive schools out of the previous mix of grammars and secondary moderns; many LEAs carried through the process swiftly (fully in Wales) but some dragged their heels; hence today's geographically messy outcome (in England).
Whilst 'Yes Minster' is parody, it is actually a very good insight into the dysfunctional working of government policy. I rewatch the episodes (and re-read the accompanying books) at regular intervals because many episodes are relevant to today's politics!
@thing47 makes the key point on this thread; academic and non-academic development is not linear - whether at school or in the workplace. Better to have processes that allow for that and not on 'one shot' opportunities, like the 11+.
To pick up @RampantIvy's point about the Progress 8 scores of grammars vs comprehensives, I have not seen large scale studies on this. However, in my part of the country, a fair number of the best performing comprehensive (all with mixed rather than 'leafy' catchments) have better Progress 8 scores than the nearest grammars schools (these being sufficiently distant not to impact on school selection behaviour by local parents in our area).
This suggests that (some) grammars take in bright cohorts but add little academic value. It probably means natural academic progression has been 'pulled forward' from Y7-9 into Y4-6 and that seems to rely heavily on access to private tutoring, as attested by the threads concerning 11+ strategising and anguish that populate Mumsnet. You could argue this is not good use of limited public money.
By contrast, a well-run comprehensive streams students and allows for movement up and down sets across subjects. My DC, all late-bloomers academically, benefitted from this. Obviously comprehensives in large conurbations can be situated in areas with challenges socially and economically but resource targeting (which already happens to a limited extent) might be a better and fairer way of addressing the issue rather than creating (or indeed persisting with) grammars.
Finally (long post this morning, I know!), @TizerorFizz makes some interesting suggestions that grammars might have been 'captured' by the middle classes in a way not envisaged back in the 1950s. Again, I have not seen large scale analysis on this but David Kynaston's excellent books on post-War Britain are rich in social data and his analysis suggests that, sadly, working class children (especially working class girls) were most definitely 'crowded out' of the grammar system.