File for future reference, lol, since you won't be in the UK this year. I've caught excellent exhibitions at both the Holburne Museum in Bath (a Canaletto exhibition) and at the Salisbury Museum on the Cathedral Close in Salisbury (a Constable exhibition).
On to Paris....
Have you been to the Marmottan? It's excellent with the number of Monet paintings on display in their lower level but is pretty well-known so you might have done this one.
Last Fall I finally got to the Museum of Hunting and Nature in Paris. I thought there would be no one there but it was the opening of a special exhibition from a fabric artist and there were at least 30 people in line at opening. Plus it was some kind of Fall school break so there were lots of families with kids. Still, I enjoyed it even though their stuffed wolf looked more like a Western US Coyote than a Yellowstone Gray Wolf. It has some animal/hunting/nature art along with taxidermies specimens of a bunch of wildlife so if you are looking for all art this would not be the place.
Musée Cognacq-Jay is also good and free as it is a City of Paris museum. I kind of laughed when I went here because there was 1 Rembrandt and 2 Canaletto's (or 2 and 1) and I thought...hmmmm...same as Nissem Camondo or some other smaller museum so that must have been the starter suggestion for 19th century art collectors, lol. First, get your hands on a Rembrandt and then try to find a Canaletto, then double it to give your collection standing among your peers! hahaha....
I'll be interested to see what others suggest!