It does seem to vary loads according to uni, and also different departments/ courses within the same uni. Some is just due to the nature of the course. First year courses designed to get the "basics" of the subject, are often big lectures (several hundred) supplemented with smaller tutorials/ workshops/ seminars/ labs where the interactive stuff goes on. Later years often seem to have more, smaller groups, with more specialised lectures - more like school classes. If the limit of students in a class is 50 for f2f, that could well impact the freshers more than others, which does seem unfair. Especially given they can mix freely in pubs etc! Though also, the nature of those big lectures is usually just imparting information and not that interactive. So there need not be any particular reason why that can't be online, as long as there is the capability for students to ask questions/ follow up with the lecturer or peers afterwards. (I was never too keen on early morning lectures as a student, so if I could have watched them from bed rather than trudging a couple of miles up a hill for 9am, I'd have possibly been up for that! If the internet gad been invented when I was a student, obviously... )
Unis have been constrained by government guidance, though, and also the glacially slow and complex uni admin systems. DHs uni had to arrange room allocations etc in May for September's intake, and they had to base the calculations allowing for 1m distancing, which was the rule at the time. On the basis that if the gov didn't relax that, everything would have had to go back online! In practice, the higher grades this year has meant they have about 20% more students than they'd planned for, so are now having to squish more students into each room. So it's lucky they no longer have the 1m restriction, and also that the way rooms were allocated has given them space to do that!
Also, I know it's very stereotyped to say, but in general the science faculties are much keener to get back to f2f teaching than arts/ humanities/ social sciences. The latter do seem much more unionised, and the UCU is very opposed to f2f, saying it's not safe, people shouldn't be forced back to campus and so on - which seems very short sighted in my opinion, but there you go. (Why a union is saying its members' jobs can easily be done by a prerecorded lecture and some Zoom calls, so asking for a reduction in the number of people needed to do that job, is beyond me!). To be fair, in general the working conditions etc in science faculties are better (more money sloshing around) so it's not entirely unexpected or unjustified. But it does seem a total lottery for the students, sadly.