Can any concept of "Separation/Balance of Power"/"Checks and Balances" not fall victim to the "Powers" forming unofficial groups outside the separation?
It certainly seems to me like "We're gonna form a club and just not go after people in our club" counters any possible "Separation of Power" anyone could put on the table.
It seems to me that the main problems are a) that most of the checks and balances being enacted by politicians, b) those that are not enacteed by politicians being enacted by political appointees.
The UK system is far from perfect, but the House of Lords are not elected politicians and are appointed by an independent commission (although some are recommended appointments, with the recommendations made by parties), and the Supreme Court who are (like all UK judges) not political appointees and not themselves politicians. And then, of course, if things get really, really far, then there's the Queen whose role is almost - but not
quite entirely ceremonial. There were serious questions very recently about whether she'd have to wield the power to remove a Prime Minister, and how legal that would actually be, but the constitution didn't end up being tested like that in the end.
As I say, it's far from perfect, but it was tested very recently when Boris Johnson unlawfully tried to prorogue parliament and was very quickly and decisively slapped down by the Supreme Court. In other words, it seems to work better than the US system, mainly because no UK political party can stack either the House of Lords or the Supreme Court with partisans. Therefore those bodies, nominally at least, remain independent of party politics.
Or, to put it another way, the Supreme Court didn't slap Johnson down because they were opposed to him personally, but because what he did was unlawful. And they didn't do it quickly because that served anybody's agenda, but because the issue at hand was extremely time-sensitive.
There are unquestionably flaws in the system (just one of which is that it's likely that now Johnson has formed a majority government he's going to curtail the power of the Supreme Court out of revenge), but I think there's evidence that it's more effective than the US system which, as you've observed, is proving fairly ineffective once people decide they're no longer obeying the gentleperson's agreement to play fair.