At that point I would either be switching providers to someone who didn't mangle my signal (because that's effectively what this is), or if that for some reason wasn't doable, I would at least install a VPN to a known secure, untampered connection (like my home router) and route everything through that connection.

Having someone intercept my signal and process it behind my back is simply not something I would tolerate from any communications-service provider I employ.

You are part of a tiny minority who has the technical interest / proficiency to do this.

To my knowledge Three UK doesn't. That's based on using them and not noticing any degradation, or the click-to-view-original-quality javascript that T-Mobile were (are?) injecting.