It might be an offence of causing harassment, alarm or distress contrary to Section 5 of the Public Order Act 1986:
(1)A person is guilty of an offence if he—
(b) displays any writing, sign or other visible representation which is threatening or abusive,
within the hearing or sight of a person likely to be caused harassment, alarm or distress thereby.
(2) An offence under this section may be committed in a public or a private place, except that no offence is committed where the words or behaviour are used, or the writing, sign or other visible representation is displayed, by a person inside a dwelling and the other person is also inside that or another dwelling.
The defendant will have two defences they could pursue:
(3) It is a defence for the accused to prove—
(a) that he had no reason to believe that there was any person within hearing or sight who was likely to be caused harassment, alarm or distress, or
(c) that his conduct was reasonable.
The court must also take into account the defendant's right to protest which is enshrined in the right to freedom of expression (Article 10) and the right to free assembly (Article 11) under the European Convention on Human Rights, as incorporated into UK law by the Human Rights Act 1998.
It is not unreasonable to be allowed to display things, during a protest, that some elements of society may find threatening or abusive. Whether the threshold for an S5 offence has been met will be determined by the court, but in this case it seems unlikely that displaying a popular figure in a noose meets the criteria, especially because they are a popular and divisive figure.
It is doubtful that a successful S5 prosecution would follow on the facts because the defendant could argue his conduct was reasonable on the grounds of exercising Article 10 and 11 rights, that the protest was peaceful until members of the Chinese Consulate dragged protestors into Consulate grounds and attacked them, and that it is part of the give-and-take of society that protests can occasionally include images and the like that may alarm or distress members of society.