Got through to a UK agent immediately yesterday to amend an Avios booking at lunchtime UK time. Last week, I was on hold for about an hour before I had to go into a meeting, but it didn't cut me off in the first instance. It appear to be pot luck - the hour last week was the longest I've been on hold throughout the pandemic - I have been pretty lucky at getting through within about 15 minutes max on the number of other occasions I have called in the past year! This doesn't really help with providing data points on where the call centre is right now but serves to show the consistent inconsistency...
P.S. Credit where it's due; the operator I got through to yesterday was super helpful and quick - knew exactly what she was doing and my changes were all complete including payment within 5 mins. The IT system was clearly working well too.