The rescue I'm involved with is. We can't rehome anyone though (no homechecks or visits) so it does very much limit what we can do. AFAIK, all kennel based rescues are still going, but cash flow is a massive problem
Don't know about small ones but Battersea emailed saying they are still available for dogs who need their help and they have rehomed lots of dogs and cats in last two weeks - still got 100 awaiting homes in their 3 centres. I am assuming they have elaborate measures in place to avoid corona spread (or maybe only homing to owners previously known to them so no need for home inspection and maybe only one visit to view selected animal rather than dropping in to see what's there - who knows?).
The rescue I volunteer homecheck for has stopped all home checks, and dogs are no longer being transported to reduce unneccessary travel. The dogs they have are being looked after, of course, but everything else is on hold for the duration.