I have no idea how it is spreading but I do know that:
Both my young adults have worked through lockdown in large retail stores, neither of them know anyone at work who has had it.
We have visited numerous hospitality places for breakfast, coffees, meals/EOTHO, since they re-opened, both local to us (current T2) and on holiday in Yorkshire and Wales. I have also eaten out a few times with friends when it was allowed. Not once have we been contacted by T&T.
I'm not convinced universities are causing the spread into the wider community.
My friends DD in T3 uni travelled home (also T3) by train last weekend for her brothers birthday. They went out for a meal. The DD is very much a party animal and I would be amazed if she wasn’t going out in her uni city. Just one example of how it can be spread into wider community. I am sure she is not alone in not adhering to rules.
Friend is a HoD at our local sec school - 2000+ kids. Every single year group has been sent home at least once in the last half term. Her comment on lockdown/schools last night was that she really didn’t feel safe in school, and felt very stressed, but recognised the exam years, and probably Y10/12 need to be there.
The school lets years 10-13 out for lunch. Large crowds of teens wander into centre of small town and gather around local chippy and 2 supermarkets, no social distancing.
I can see why they don’t want to close schools and unis, but am not convinced that closing everything else but them will lower the numbers. Time will tell.