Brit here, also blocked, with a similar message, but to be fair, if that website had any good ancient artefacts on it i'd be over there to steal them in a flash, so maybe it's for the best.
Since it is a government website, I suspect that foreign IP have a strong historical correlation with mischief.
Particularly because mostly forgotten webpages like this one are an attractive nuisance for black hats, banning foreign IP’s was probably a best-practice argument.
There’s also the possibility that a sudden uptick in activity (HN slashsot effect) triggered an internal mechanism.
And politically, I suspect there wouldn’t be a lot of pushback to any of it.