I don't know if that is the key difference here.

Madrid insists they couldn't allow Catalonia to become independent even if they wanted to.

Many people in Spain outside of Catalonia are resolutely opposed to Catalan independence, under any circumstances. By contrast, most people in UK outside of Scotland don't really care, and even the vast majority of those opposed to it would be willing to accept it if a referendum voted in favour. (Even those opposed to a second independence referendum, their argument is "too soon" rather than "never again"). In this regard, Spain is culturally more like China than the UK – pro-Beijing people get terribly upset at the idea of any territory claimed by the PRC ever becoming independent of it.

> Madrid insists they couldn't allow Catalonia to become independent even if they wanted to.

Which may or may not be a correct interpretation of the Spanish Constitution, but what the US Constitution does or does not allow regarding either secession or Congressional grants of independence is not really germane one way or another. They aren't even products of the same legal tradition such that analysis of one might be illuminating on the other.

Spanish opponents of Catalan independence repeatedly use the argument "Our constitution doesn't allow a part of our country to become independent; but in that regard our constitution is no different from those of many other countries".

So the question of what other countries' constitutions allow is relevant to the debate.

And the comment I was initially responding to was suggesting that the UK could only allow Scotland the choice of independence because it has an unwritten constitution. Explaining how other country's written constitutions could allow grants of independence to parts of the country is a relevant response.