Toxic mentally??!! WTF? You have taken my words the wrong way pal. It was not an arrogant 'Im from the UK you know!'
Easy. I'm not your pal, friend. I just thought it was worth pointing out. I'm also a UK national but it's worth noting that being from UK doesn't automagically grant us some sort of super rights. That's why things like Netflix in the US differs to UK. There's various moving parts at play and your message comes across as if to suggest that problems of region specific keys do not exist in the UK as we're above that kind of problem. Realistically that's a ludicrous notion! Anyway, I've said my piece! Moving on.
It sounds like Humble have given you a key that's not redeemable in the UK for whatever reason (maybe it was at time of issue but not at time of redemption or maybe it's just a clear error on their part). Either way, I'd contact humble support and explain to them the issue as it's for them to resolve rather than steam. It's not clear which support you've already contacted.
Also before I go on: I work in law, but not civil law and so take what I say with a pinch of salt and read the legislation yourself first. You will most likely be able to invoke your rights under the Consumer Rights Act if they give you any pushback which they most likely will. Quote that you're a UK citizen and explain that your rights under the Consumer Rights Act 2015 extends to digital goods purchased and that you would like to invoke those rights as the keys are not redeemable in your country and hence not fit for purpose or as described, breaching s35 and s36 of the CRA 2015 and that you would like to invoke your right to a repair/replacement/refund/price reduction (pick one). This is of course assuming that you made the purchase legitimately within the UK (not from another country or whilst using a VPN) and that the error is humbles fault.
PS. I'd suggest redeeming all of the games when you get them as humble have been known to run out of keys for games and this can potentially cause issues like this OR cause them to simply not be available. In other words, get 'em while they're hot.