I haven't seen the BBC series, so I can't get specific about the accuracy of Gregory's portrayals. But on one specific point you mention, Elizabeth of York's relationships with Richard and Henry: there's exactly zero reason to believe that there was any romantic or sexual relationship, consensual or otherwise, between her and Richard. The closest thing we have is that his opponents spread rumours that he was planning to marry her for political reasons, but as far as we know, that was just propaganda á la Pizzagate, throwing mud in the hope that it would stick: when it comes to solid fact, all we know is that Richard flatly denied the rumours, and that he spent a fair amount of time and effort working on betrothing Elizabeth to other people. There's also zero evidence that Henry was a horrible abusive rapist: yes, it was an arranged marriage and there's no reason to think that Elizabeth had any desire to marry him, but arranged marriage for the sake of political alliance was absolutely the norm among nobles at the time, and there's no evidence that theirs turned out any worse than most. On the contrary, there's evidence that by the end of their marriage, at least, Henry genuinely cared about her and was devastated by her death. So on those points, at least, Gregory is working purely off imagination and what would make a good story, not off the actual evidence. This implies to me that she may well have done the same elsewhere.

The fact is, none of these people's documented actions stack up well against our modern standards of good behaviour. All of them could be what we would consider ruthless and brutal. Take Richard: when he knew Henry was headed to attack him, and wanted Lord Stanley to come in on his side, he took Stanley's son hostage and threatened to kill him if Stanley didn't play ball. (Stanley apparently sent back a message saying 'I have other sons', which must have made for some interesting family dynamics afterwards.) That doesn't exactly fit our standards for a good guy. Or Edward: he had their other brother George executed for treason, which, even though George had in fact committed treason, feels kind of harsh. Or Henry: basically his first action as king was to date his reign from the day before the Battle of Bosworth, which meant he could declare all Richard's supporters to be traitors and scoop their lands into his possession, which is pretty slimy. All of them did a fair amount of executing people, some of those on more flimsy pretexts than others, and a sizeable amount of political scheming. Nobody here is a romantic hero. They're living in brutal times, surrounded by political and physical danger, and they're surviving by being brutal when they think it's needed.

On the other hand, again by the standards of their time, none of these people was a terrible king. Edward was probably the least successful in political terms. He married Elizabeth Woodville for love, thus scuppering the politically valuable marriage that people had been arranging for him, and then he let his wife's family overrun the court to the point where the resultant tensions destabilised his entire regime, got him temporarily booted off the throne, and contributed to the mess after his death. He also had a habit of asking for money 'to fight this war that we totally need' and then spending it on his own stuff.

Richard only had two years on the throne, so it's hard to say what he would have done with a reign as long as the others', but he was a lot less flaky and more practical than Edward. A lot of his decisions focused on making things work better on an everyday, concrete level: a Court of Requests where poor people could get their cases heard cheaply and quickly, having the laws translated from French into English, protecting suspected felons from having their property seized before they'd been found guilty.

Henry could be pretty scuzzy, finding all kinds of sneaky and unethical ways to squeeze money out of his subjects. But he did have probably the broadest grasp of large-scale statecraft and what was needed to unify and stabilise a country and position it strongly in international terms, and he took big steps in that direction.

The thing is, the York brothers are a lot easier to romanticise than Henry. Edward was handsome, cultured, charismatic, an excellent military commander - and if you weren't one of the people who had to clean up the mess, the whole 'secret love marriage to a destitute beauty from the other side of a civil war' thing is pure romance fodder. Richard wasn't a charismatic stud, but he was universally acknowledged as a fighter of great courage and a military commander of good sense and intelligence, and he was loyal to Edward through every up and down, even when Edward looked like being a complete washout and Richard's loyalty took him into exile. Also, Richard was traduced by history, demonised by his killers way beyond what the evidence justifies, and the misunderstood underdog is always romantic. Henry, meanwhile, was a little weenie who sent other people into battle to fight and die to put him on the throne, while he hid safely behind the battle lines with his bodyguard to protect him, and then he spent his kingship coming up with sneaky ways to wring money and power out of anywhere he could find them. He's the kind of guy who today would work in claims for an insurance company and find hair-splitty little ways to avoid paying out, while nudging other people to file complaints about the boss with HR so he can apply for the post when the boss gets fired. There's not a lot of romance to be found there.

So it's not that the York brothers were heroes and Henry was a villain - or vice versa. It's that we don't have an awful lot of evidence about these people, especially when it comes to emotions and personal relationships. We can infer some things with reasonable certainty (it's clear from his actions that Richard didn't like the Woodvilles one bit, for example), but others are mysteries - we have no idea how Elizabeth felt about Richard, say. So if you want to storify history in order to write a book or a series about them, you're going to have to fill in the gaps. And if you're looking to write romantic heroes and cunning villains - which is the simplest way, if not the most interesting, to establish dramatic tension - the York brothers lend themselves more easily to the first role, and Henry to the second. If there's justification for the portrayal, it's literary rather than historical.