Seizure, and obtaining of passwords of, communications devices.

In England a few years ago, a woman was arrested for allegedly making transphobic tweets on social media. Police told her that the Regulations of Investigative Powers Act (RIPA) meant that she had to disclose the passwords of her communications devices. On legal advice, she complied. She was not charged. She is suing the police.

What is she suing the police for?

I know that the UK does not have a written constitution but, given what is traditionally understood as 'fair play' in Britain (e.g. the Magna Carta), how could Parliament have authorised police to go to people's houses and arrest them for allegedly hateful online posts and seize their communications devices (i.e. laptops, iPhones, iPads) without asking judges to grant warrants for the arrests and seizures of the devices?

Parliament, in the UK, can legislate anything. It can legislate to override, modify or repeal Magna Carta, and in fact very little of Magna Carta remains in force.

(Worth adding that arrest without a warrant is and always has been extremely common in the UK. Very few arrests are carried out on foot of a warrant. Most arrests are carried out on the basis of reasonable suspicion of involvement in an offence. It's the seizure of the communication devices, and even more the obligation to provide the password, that's more unusual.)

Anyone looking to challenge this legislation is almost certainly arguing that it's a violation of the UK's obligations under the European Convention on Human Rights. relying in particular on ECHR provisions dealing with the right to privacy and the right against self-incrimination.

Furthermore, regarding the compelling of suspects to disclose passwords, do police and prosecutors in England not consider the possibility of a suspect forgetting a password?

That's a good question. I don't know if anyone has been prosecuted for not disclosing a password which they say they have forgotten. I'm speculating, but it may be policy not to prosecute in such a case, if the authorities don't want a court ruling that says this is a good defence.

If the authorities really want to read your messages without your getting your password, they mostly can. It just takes a bit longer.

Her civil case is ongoing and so she can't say much about it.

The grounds on which she is suing would be a matter of public record. (But I only ask out of curiosity. It seems to me that if she handed over her password at the time, on legal advice, and subsequently felt she was not legally obliged to do so, she might sue her lawyers, rather than (or as well as) the police.)

As for not disclosing a password, a man who was suspected of the murder of a girl was jailed for that. He didn't claim that he had forgotten it. Eventually, he was found guilty of the girl's murder.

He's not a character who attracts sympathy, obviously, but the case does make the point that illiberal measures that are said, when introduced, to be justified by the need to address a threat of terrorism very often come to be used by the authorities in circumstances that have nothing to do with terrorism.

Indeed. But it's pretty common for terrorism to be offered as a political justification for introducing illilberal laws when the laws themselves don't actually mention terrorism.

(In fact this case is an example. When the Regulation of Investigative Powers legislation was first instroduced in the UK, by the Blair government, it was said to be necessitated by the phenomena of terrrorism, trans-national organised crime and paedophilia. But here we see it being used in a situation that has nothing to do with any of that.)

That's kind of my point, PA. It's common for a government introducing a controversial law to justify it by saying "we need these powers to deal with problems X, Y and Z", but the law itself, on its own terms, is not confined to problems X, Y and Z, and, once enacted, it is used in relation to entirely different (and much less serious) problems A, B and C.

Back in the OP, you raised this question:

. . . how could Parliament have authorised police to go to people's houses and arrest them for allegedly hateful online posts and seize their communications devices (i.e. laptops, iPhones, iPads) without asking judges to grant warrants for the arrests and seizures of the devices?

The answer is: Parliament was told that these powers were necessary to combat terrorism, transationational organised crime and paedophilia. They were not told that the powers would be used to combat "allegedly hateful online posts".

The lesson we draw from this, I think, is that when government seek sweeping powers, and offer an assurance against abuse of those powers along the lines of "you can trust us — the innocent have nothing to fear", it's not necessarily true that we can trust them and that the innocent have nothing to fear.