Thanks for the reply. A bit of a (long) update:
I called Tesla the next morning and was told that any brake issue couldn't be related to the MCU/FSD/CCS upgrades i'd had, and so it must be a new problem, and since the car is just out of warranty I would have to get it to Tesla and have it fixed at my own cost. Since it was at home, they wouldn't arrange a tow for me (even at my cost), which was very annoying. I spent a while trying to find someone to tow it to no avail (it turns out all towing companies in the UK seem to be very busy right now, I guess everyone's realised their cars don't work after lockdown!).
When I checked the car again, the brakes were also no longer stuck on. I ended up calling the AA out, and the technician inspected the brakes and discs and did some basic diagnostics. He confirmed that it looked like the brakes had been stuck on (discolouration on the front left disc), but he couldn't see any problem with the brakes any more and suspected a software problem. We tested the brakes briefly and they seemed fine, and drove round the block and did a few emergency stops, again no problems, all felt back to normal.
Since I couldn't get the car back to Tesla until after the weekend, and the brakes seemed to be working ok, the next day I tried a short journey nearby at low speed. Everything was fine for the first five minutes, and then lo and behold suddenly lots of alerts flashed up on the dash and the brakes suddenly failed! They had very little stopping power unless I really pushed hard on the pedal, fortunately I was doing about 5 mph at the time on a small road and could pull over. I called Tesla roadside assistance, they confirmed they could see the brake issues and asked me to check under the car for any liquid. Sure enough, there was a lot of liquid coming out (like someone was pouring a can of coke out slowly, in terms of quantity and rate), which I assumed was brake fluid. This time Tesla arranged for a tow and it was taken to the service centre.
I've just had an update from the service centre, and they told me that there was a faulty brake switch which was fitted as part of the MCU/FSD/CCS upgrades, so the upgrade was actually the cause of the issue! A faulty switch makes sense for the initial symptoms (brake stuck on, car reported both pedals pressed when they weren't), and could explain the subsequent brake failure, but it doesn't explain the liquid leaking from the car. Tesla say that the brake fluid is full, they can detect no leaks and that the liquid must have been AC run-off. To me that just seems unlikely, I know that water can drip from the AC, but I haven't ever seen water running from the bottom of the car like that. It also seems unlikely that it would condense so much water over the course of a very short drive in not particularly hot or humid conditions. It was hard to tell whether the liquid was water or something else like brake fluid or coolant, since it was pouring onto a dirty road surface.
I guess it's just worth noting that a seemingly unrelated upgrade of the MCU, FSD computer and charger requires changing a brake switch, and that single switch failing can cause brakes to fail in a fairly catastrophic way.
Does anyone have any insight into whether AC run-off is plausible for all the liquid that was leaking? I guess if the brake fluid reservoir really is still full and they can find no leaks, it can't really be brake fluid. I guess there are other coolants and liquids in the car though.
Tesla are doing another round of checks and I hope to pick the car up tomorrow. It sounds like they won't charge me since the problem was caused during the upgrade process.
