I wouldn't say that the mushroom exhibition - three small rooms at Somerset House - is terribly vaut le détour unless you happen to be in the courtyard already and it's a very short détour. (Though why would you be, until the Courtauld re-opens?)
But I did like the tiny display of Alice editions, especially this very anachronistic one from 1932.
Shoes and lampshades made from mushrooms, though, were hideous - and the lovely Beatrix Potter watercolours of fungi were hung too high, which was a shame.