Except that maðum actually means treasure and maðumhus is treasury. (Tolkien was, perhaps, engaging in a bit of wordplay on this one, equating the old relics the hobbits kept with treasure.) A variant of laf, which is remnant or heirloom--lafhus--might be better.
But I see nothing wrong with using a modern borrowing for a concept that didn’t exist in Anglo-Saxon times. But in this case, I think I would have kept the title as “British Museum.” After all, that is the formal title of the institution and doesn’t need to be translated.
And anyway, it should be “Bryttisc Bochus,” as the manuscript is in the British Library, not the British Museum. (A common error made by using old sources from when the British Library was part of the British Museum. The institution of the British Library was not created until 1973, and the current building was not opened until 1998.)