The wiki page for LiDAR sources for the United Kingdom lists a few WMS endpoints that JOSM seems to load OK (if slowly).
Are these OK for OSM use? The US equivalents seem to be available in JOSM’s imagery list natively, and the lack of that has me concerned that there may be some caveats or regional variation on usability.
There are a couple of streams I know of where the DTM especially would be a very useful source for seeing through the treecover, but I don’t want to use anything that hasn’t been reviewed and confirmed to be OK.