I know I’m in the minority with my opinion, but for situations like this I feel like an unrestricted (eg doesn’t require Avios availability) single class upgrade is fair compensation as the OP effectively received a downgraded experience. The corresponding number of Avios is not sufficient given the challenge in using them.

I often use the example of buying a computer from Apple. If you get a MacBook and the space key doesn’t work, Apple doesn’t say “here’s an iTunes gift card, you’re stuck with a broken key on your new machine, but it mostly works fine” nor would they say “you paid full price for a 15” model, but here’s a 13” model and a voucher for 100 quid”. They would make it right by replacing or fixing the issue. With the perishable nature of airline seats the closest option would be what I proposed above. Of course the abuse of such a system is why we don’t have such a system in place today, and I don’t have answers for what is a significant enough issue to offer the compensation I propose.