I'm not sure these problems are completely unrelated to brexit. I haven't really gone through the existing thread but I can't help but think that the brexit associated changes has messed up a lot of things.
There are multiple reasons:
Changes to VAT and duty processes = not Brexit
Crossing the EU/UK border = Brexit
Staffing = Covid, numbers of staff available and staff vs distanced capacity (and perhaps also Brexit if EU staff left, or EU occasional staff left)
Border stop/starts = Covid & Brexit
Then if you add NI into the loop then you have the combined mess of Brexit, Good Friday Agreement .... and drug traffickers who get caught out by a virtual border
Every carrier has had problems relating to the combination of paperwork & process changes, shortages, throughout capacity etc.
A minor problem easily turns in to a backlog with more items coming in all of the time, and unless you can put out more then is coming in then you have the situation compounding itself.
UPS has had the worst problem, and it may also relate to a particular depot