I'm not seeing any evidence there that there has to be a "disciplinary route" for dismissal to not be 'automatically' unfair. Where are you seeing that? It could be ruled unfair, but that's for the tribunal to decide, and you have no right to that tribunal until 2 years is up.
As such the chance of being able to take this through the official channels seems minimal, and that's been the experience I've seen other people get (not from games, but in UK employment law generally).
By all means the employee can try ACAS and see if they're willing to touch the case. Ultimately though, anyone joining the games industry in the UK and thinking that they can refuse the opt-out everywhere they go is likely to find their career somewhat hampered, sadly.