I think in an ideal world, lectures would deliver value for money actually be useful in providing a learning experience that you couldn't get yourself just by reading notes/textbooks at home, and hence you were vastly more likely to do well in your degree if you attended them than if you didn't. Following that, it wouldn't make much difference if lectures were mandatory or not - nobody would want to miss them.
Currently, it seems that the only reason lectures (or a certain percentage of them) are mandatory is because if they weren't, it would quite clearly expose how useless some of them can be, leading to valid questions around what the university is actually charging £9k per year for.
I used to have 12 hours of lectures per week, and 20 weeks of lectures per year (studying Maths at UCL). This suggests that each student is paying around £38.50 per hour of lectures, which is frankly ridiculous, considering that most lectures consisted of nothing other than the lecturer reading from a script, and that you could probably get proper, personalised, one-on-one private tuition for about that much.
I don't understand why university qualifications can't be the same as GCSE and A-Level qualifications - where the exams are set by an independent examining board, and the job of the university is to do their best to educate you so that you can do well on them. If you can do well on those exams without attending lectures or indeed without attending university at all, good for you - you didn't need their help.