The requirement for reasonable suspicion is real, and if the police cannot establish this then they are in breach of the law.
Yes, although the "reasonable suspicion" required for an arrest in the UK law is a much lower standard than the "probable cause" required in the US. And if the police decide to arrest you, you cannot resist arrest on the grounds that you think they do not have the required reasonable suspicion, so you still get arrested. You can then be held for 24 hours, under conditions which are unlikely to be pleasant, before they have to charge you or let you go. Of course, if the police did not have reasonable suspicion, the arrest and any subsequent detention were unlawful, but there is no crime of unlawful arrest: it is a civil wrong and your only recourse is to sue the police. So the requirement for reasonable suspicion is not much of a protection, in practice, if you are, to put it delicately, the kind of person likely to arouse the unreasonable suspicion of the police.
The Terrorism Act, 2000 was originally passed with the now notorious s44, which allowed the police to define areas where they could stop and search
without reasonable suspicion, and seize "articles of a kind that could be used in connection with terrorism" - ie, anything whatever. Several security laws applied to Northern Ireland also allowed the police to arrest anyone they suspected -
not reasonably suspected - of terrorism.
The fact that those laws are no longer in effect is beside the point: they were passed, and similar laws can and will be passed again if they suit the convenience of government. The point is simply that the idea that freedom, and in particular freedom of speech, is deeply entrenched in the UK and US is false.