@Gavin52 you originally wrote: "I feel this is an important matter of consumer rights and one that I feel could be breaking the law in the UK not to advertise the terms of discount at the point of sale and it should be treated with seriousness."
You offered a 'first-3 guest' discount, but the placement of it was not right with the offer itself in proximity but at the very bottom of the page, thus not adhering exactly with 'discount law'.
Again, I would have given him the discount also and moved on and not wasted a nano-second. That is what this host would have done, and if your originally intent was to extract other host's opinion, you got mine.
Yes, all things are subject to improvement; perhaps you should write to Airbnb direct (via feedback) and maybe they will be in agreement and make the change. Nothing to loose.