The new emission regime might be something to do with it, manufacturers shifting stock before 1 Sept and having limited stock approved for sale after 1 Sept - some new car lead times are long because the testing takes time.
French brands are suffering, PSA and Renault seem to be doing badly on September sales, not too bad on YTD (see the tabs on the attached link). With DS down by 41% YTD, all those "exclusive" Citroen dealers approved (or compelled?) to have a separate DS showroom might be wondering why they bothered.
https://www.smmt.co.uk/vehicle-data/car-registrations/
If anybody spots THE 68 plate Chevrolet, do tell us
