When someone says X has Y property they don't normally mean it has Y property by definition or law of physics, they mean that almost all instances of X have Y property, to the point that you can effectively treat Xs as all being Ys and you're unlikely to run in to something in your life that contradicts this.
It's like saying "trans peoples aren't mass murderers". Yes, there may be an exception, but for pretty much all practical purposes it's true.
They’re not, and pretending that they are is disingenuous. Compared to American police services, they may seem fine. And the nominal theory of policing in Britain (Peelian principles) is more sound than the theory of policing in America…but that does not mean that police forces in the UK are free from power-trippers, abusers, and even systemic bias against whole classes of UK citizens and residents.
UK police also tend to improve and institutionally learn over time as we get inquests and some of the recommendations get implemented.
Maybe you’re thinking of the fine upstanding members of the Greater Manchester Police who destroyed evidence and did not turn over evidence to the prosecution (to provide to the defence) which have cost Andrew Malkinson 17 years of his life: https://www.theguardian.com/uk-news/2023/jul/26/appeal-court...
Or are you perhaps thinking of the upstanding members of the South Yorkshire Police involved in the disastrous crowd control and subsequent cover up of said incompetence around the Hillsborough incident?
It seems unlikely that Northern Ireland’s police services are fine, either, given that sectarian violence and military involvement rampant in the Troubles (caused mostly by hard-liners and reciprocal incidents, and the Iron Lady’s stance helped everything as much as anything she ever did, which is to say not at all), but that may have changed since the Good Friday Accords took effect. It may also not have changed, given that sectarianism is still fairly rampant and the DUP are acting like spoiled children.
Maybe the Welsh and Scottish police are fine, but I am sure that a few minutes of searching could find examples of institutional issues for both of them.