From Chris Waigl, passed on by Chris Hansen:
The problem begins with the subject, a longboat full of Vikings. The (syntactic) head of this phrase is certainly longboat (and that’s what determines agreement on the verb), but it’s functioning here semantically / pragmatically as as an expression of measure, much like a collective noun. So the question is whether the subject is “about” a longboat or “about” Vikings. (Animate beings, especially humans, are especially favored as topics, ceteris paribus, so we should probably look to the Vikings.)
At the same time, the first sentence introduces the British Museum and the Palace of Westminster, implicitly (but quite subtly) introducing the Members of Parliament as entities in the discourse, though probably not as the topic.
Then we get the second sentence, which is clearly about Vikings (uncivilized, destructive, and rapacious), not boats (or the Members of Partliament, for that matter).