Top 5 computer science and mathematics departments in the UK

Excluding Oxbridge, please give me a top 5 list for the "Computer Science and Mathematics(joint course)" course. I realise there is no factually correct answer here, but I nevertheless believe this should be a good starting point in shortlisting my unis. Thank you.

What is your definition of "best"?
Outcomes of their graduates?
Job prospects of their graduates?
Location?
Quality of facilities?
Quality of teaching?
Prestige?

I imagine that in your mind, you are thinking of prestige because you mentioned Oxbridge. However, you might be shocked to find that on the likes of job prospects or quality of teaching which at the end of the day are what actually matters, the answers you get back might surprise you. For example, University of East Anglia, Surry and Liverpool Hope all out perform Oxbridge on job prospects
https://www.thecompleteuniversityguide.co.uk/league-tables/rankings/computer-science?sortby=graduate-prospects

Just to say right now, no employer cares where you went. They only care what you know and what you can do and how you fit into their business. The colour of your tie counts for nothing.

I can't help but think that I'm gonna end up underperforming/not pushing myself enough/getting bored if the university is lenient/the students are not serious enough.

Welcome to the grown up world. That is on you, not the university. And ironically the better universities are likely to be more lenient. At least in offering no support over and above a promise to withdraw you if you don't perform.

Good luck!