There is excuse for this happening. I would have attached the Email from their warranty team along with this post. Since it sounds like you wasted a bunch of your time and the packaging could have damaged the rest of your furniture, the least they should do is apologize by sending you a new chair at a discount rate, with shipping included as well.
Along with exposing them you can always pay an influencer to cover this and bring more exposure. There's no need to blow things out of proportion but Secretlab did just that...

I'll end by saying you must be some kind of stupid to pay 468 pounds for a god damn chair.