The autumn equinox falls around the 22nd, and from that point the nights are longer than the days. Britain loses daylight faster in September than in any other month except March gains it — roughly four minutes an evening, which adds up to two hours over the course of September 2029.
September closes the third calendar quarter of 2029 on the 30th, and 30 September is also the financial year end for a substantial number of British companies. The tax year, by contrast, is not quite six months old — the two systems are as far out of alignment here as they ever get.
September 2029 runs to 30 days and closes the third quarter of a year of 365 days. One quarter remains, and it carries the clock change, both November commemorations, the Christmas break and the turn of the calendar year itself.