Some interesting figures from the CAA website and easyJet's own reports (well, I think they're interesting for what they tell you, anyway).
For the UK AOC only, July 2023 figures show:
A319s operating at 88.4% of seats filled
A320s operating at 87.1% of seats filled
A320Ns operating at 88.3% of seats filled
A321s operating at 89.5% of seats filled
Total is 87.9% for all aircraft variants
This is a pure "bums on seats" measurement. If you look at the reported load factor (seat km used over seat km available or RPK over ASK in airline speak) on which the CAA also report, the load factor is 91.99%. What becomes clear from a comparison of those figures is that their performance on the longer routes is better, and they must have a number of short sectors which are acting as a drag on the overall seat factor. The A321s are generally flown on the longer routes as well, and have a slightly higher seat factor than the rest of the easyJet UK aircraft. easyJet reported a network-wide "bums on seats" seat factor (so UK, Austria and Swiss AOCs all combined and which include seats sold to passengers who didn't travel) of 92.5%.
August figures for the UK AOC:
A319s operating at 90.2% of seats filled
A320s operating at 89.0% of seats filled
A320Ns operating at 89.9% of seats filled
A321s operating at 91.8% of seats filled
Total of 89.7% for all aircraft variants
Again the network RPK/ASK measure produces a figure of 92.6% for that same fleet in August - so the disparity is there again. The easyJet reported network seat factor of 92.5%.
Before anyone says you'd expect this in July/August, don't forget a) how much capacity easyJet deploy from the domestic "business" markets onto leisure routes during this period anyway and b) the trend is much the same for May and June anyway.
The message in here is that the A321s on their long sector runs are doing well, but they have some short-sector flying in the network which is performing quite badly to be able to make a visible dent in the RPK/ASK versus seat factor comparison in this way. It does make you wonder about some of this headlong rush into Belfast City, Southampton and others - although you can't tell what is working and what isn't, some short sector flying very obviously can't be working as it should be. But yet they keep doing more of it!