Typical,
I too was a Executive Gold member that gave up on them because of their crap customer service. Too thick to understand that their failure to right their wrongs cost them tens of thousands of £s in lost revenues.
Well I've retired now and no longer travel for business much. But I still never use that airline.
Pity for the crews as I almost always has great service on board. But their ground staff were nothing to write home about and their customer service team didn't have a clue.
Seems nothing has changed.
So my advice, escalate your claim to the CEO's office. Obviously he won't read it but someone in his offices will. Set out your complaint clearly and without (too much) emotion. Point out that if you had been offloaded it would have cost them €600, plus the cost of hotac and meals until your next flight. Most importantly tell them what you want to make you happy. Don't be subtle, they don't understand subtle. If you want £500 say you want £500, if you want a free upgrade on your next longhaul, say so.
I wouldn't bother going to the CAA as they don't seem able to deal with customer concerns. Look at you small claims court options instead.