Computer Misuse Act 1990?
There are three major issues with what MS is doing, and I think it may be worth getting legal advice.
1 - This is not a patch, this is a major OS change. It thus has simply no place in a patching system as it interferes with the proper operation of a system. The fact that MS doesn't make that distinction should tell you something about their idea of patching, upgrades and general system maintenance.
2 - The install happens AGAINST the explicit wishes of the holder. Given (1) I think it can be declared unwanted interference with a computer, even if the user has not explicitly hunted down the tickbox, unticked it, hidden it, sacrificed three goats (or, if not available, traffic wardens) and sang Kumbaya before running a patching process. This puts it IMHO firmly inside reach of the Computer Misuse Act 1990, and I hope that a larger company gets hit by this so they can sue their socks off. Come to think of it, isn't this a criminal offence? In that case, it's your taxes paying for it so go right ahead and complain.
3 - Windows 10 requires (as far as I know) the acceptance of an entirely new set of T&Cs which contain conditions that may have been invalidated by the recent Facebook vs Europe death of Safe Harbor. Ramming this down people's throat without their explicit permission is again legally questionable.
Message to Microsoft: stop this. NOW. Not only is this the road to legal action, it is also pissing off the last customers you still have.
Message from OSX, BSD and Linux users: please, please, PLEASE continue. More!! Why bother asking, Just Do It™. Oh no, that's Nike.