Once you've been through each stage of the university's complaints process, you can take your case to the Office of the Independent Adjudicator for Higher Education (OIA).
The OIA has guidance on what they would look for if a student complained to them. They will review what the university promised in its contract and promotional materials, and what students could reasonably expect in terms of contact hours and learning opportunities. They will then assess whether the university has met those expectations and whether suitable alternatives were provided during the strike. If the university failed to meet expectations, the OIA will consider whether the university made any efforts to mitigate the disruption.
The OIA has published some real-life case summaries, so you can see some examples of cases and their outcomes. Several complaints against universities have been upheld in the past because the university hadn't made up for lost learning opportunities. This is a quote from one case summary as an example:
We decided the University had taken appropriate steps to minimise the academic impact of the industrial action. However, it had not taken any steps to make up for the lost teaching hours and the learning opportunities they represented. We explained that, in our experience, most students don’t study at higher education providers purely to gain a qualification. Students who expect to learn about a particular subject but who don’t receive the teaching they are supposed to get are not adequately compensated by the provider undertaking not to test them on it. It is reasonable to expect providers to make some attempt to make up for what has been missed, but this doesn’t have to be like-for-like replacement teaching hours.'
Something else they consider is whether an individual student's needs have been met. One case outcome states:
The University had not recognised the individual effect of the industrial action on the student, and had not considered whether it would be reasonable to take further action or to provide additional support because of the student’s individual circumstances. In particular, the University hadn’t thought about giving the student a deadline extension for their other modules so that the deadlines were spaced out; the student had explained at the time that grouping deadlines together was increasing their anxiety.'