hmm, okay. I think it's difficult to tell if the fungus is the one which is found in Spain. It's most likely, because the ants come from there and the fungus was spotted there too.
Otherwise it might be a fungus you became through feeding. The insects carried the spores and your ants weren't used to it (so Lasius niger in UK have no problems with a common fungus) That might be possible ... I don't know the fungus in UK and which of them might be dangerous for this specific Ant species.
Antstore is controlling their ants they say ... but I don't think they can exclude the possibility that this might happen. Spores you won't see so easy and there's mostly an incubation time before the fungus will start to grow. Nothing is 100% even if you give your best
Without telling which fungus that is, it is difficult to tell who's to "blame" for. As long that is the only case of a fungus infection nobody can tell with just photos.
I'm sorry for you colony, next time it will surely work fine. I'm sure!
greetings
... or maybe it isn't a fungus. it might be moist ... spanish ants aren't used to British rainy weather

okay, i did't want to joke ... just a little gallows humour