"I would bet you'll be the only one on the flight"
Absolutely not true. People have been cancelling some travel recently due to the new cases in Beijing and elsewhere, but every domestic flight I've been on in the past few months has been at least half full, with many completely full.
The test is now available at most large hospitals in Kunming. I got my last one at Boya a few months ago, and it was a very painless experience. Make sure you get a red stamp from the hospital on your negative test results, because... China.
I have not heard about any requirement for UK citizens to have a test for travel in China, but the specific rules will vary from one city to the next.
Since all this started, I have made a point of calling any hotel where I have a booking to confirm that they still accept foreigners, and whether they require any further documentation beyond the green health code.
You are correct that no new tourist visas are being issued at this time.
Your only option right now is to give up the M visa, and get as many extensions to the S2 visa as they are willing to give you. The only way to hold onto your M visa right now would be to return to your home country and wait things out (and it could be a while).
Once things cool down and China resumes normal travel, you can go back to your home country and apply for a new tourist visa, or try for the M visa again.