Attendance is registered by smart card, the system isn't perfect so you often have to sign in on paper as well.
I don't know what you are studying but the AHP (allied health professions - occupational therapy/physiotherapy/podiatry) require 80% or greater attendance to be allowed to go on placement.
You are expected to attend all classes, in my lectures (OT) we are allowed to ask questions and sometimes have discussion sessions.
You should only miss classes if you are ill or attending a medical appointment (both require notifying the tutor). There is some leeway if somebody lives off campus and the weather is bad as long as they notify the tutor and they read through the lecture that was missed.
Most classes are quite small and the tutors do get to know everybody in the class so they would know if you were off.
The university does kick people out if they don't attend but this is after quite a few steps which are taken by the university, starting with the personal academic tutor (PAT) and the pastoral service so try to find out why the person is having issues with attending.
Here are two FAQ's
Attendance Policy and
Electronic Register SystemTL-DR - You need to attend all classes (lectures/seminars/practicals) and the university does record attendance using electronic and paper systems.