It will not be the first such protest in London - e.g. About 100,000 turn out in London for pro-Palestine rally | Israel-Hamas war | The Guardian 21 October 2023
The Metropolitan Police Commissioner (Sir Mark Rowley) has clearly decided that the legal conditions required to actually prohibit a procession are not met - Armistice Day demo will go ahead - Met Police (thelondoneconomic.com)
Prime Minister Rishi Sunak has said that the planned marches are "disrespectful" and that his job is to hold the Metropolitan Police Commissioner accountable for his decision - Sunak to hold Met chief ‘accountable’ for decision not to ban pro-Palestine march | Metropolitan police | The Guardian.
Precisely what the Prime Minister had in mind when he spoke of "accountable" is not clear. The Commissioner is supposedly independent of the government regarding operational decisions.
1] The European Convention on Human Rights Articles 10 and 11 are concerned, respectively, with with Freedom of Expression and Freedom of Association - Article 11: Freedom of assembly and association | EHRC (equalityhumanrights.com). These rights are "convention rights" for the purposes of the Human Rights Act 1998.
2] The Public Order Act 1986 (Part II) deals with Public Processions and Assemblies. The rules relating to each of those forms of protest have similarities but there are some significant differences.
3] The Police must normally be given advance notice of planned processions (section 11) and section 12 permits the Police to impose conditions on a procession. The "Senior Police Officer" must reasonably believe that the procession may "result in serious public disorder, serious damage to property or serious disruption to the life of the community." Any conditions must appear to the Senior Police Officer to be necessary to prevent such disorder, damage, disruption, impact or intimidation and conditions may be applied to the procession route and may prohibit a procession from entering any public place specified.
A power to prohibit processions is in section 13. Section 13(4) states - "If at any time the Commissioner of Police for the City of London or the Commissioner of Police of the Metropolis reasonably believes that, because of particular circumstances existing in his police area or part of it, the powers under section 12 will not be sufficient to prevent the holding of public processions in that area or part from resulting in serious public disorder, he may with the consent of the Secretary of State make an order prohibiting for such period not exceeding 3 months as may be specified in the order the holding of all public processions (or of any class of public procession so specified) in the area or part concerned."
Hence, there is a power to prohibit processions but the consent of the Secretary of State is required before the Police can make an order. That is, in practice, the Home Secretary. Constitutionally, the Home Secretary would be answerable to Parliament for any part played in imposing a prohibition.
4] Public Assemblies are the subject of section 14. It is a lengthy section enabling conditions to be applied by the Police to public assembles with a view to preventing serious public disorder, serious damage to property or serious disruption to the community.
That appears to be the relevant law. The Metropolitan Police Commissioner is within his legal rights to assess the plans for any protest and it is for him to decide whether conditions will suffice to meet any concerns about possible disorder. With the Home Secretary's consent, an order prohibiting processions is possible - (subject to section 13).
The Terrorism Act 2000 proscribes certain organisations and one of them is Hamas (Harakat al-Muqawamah al-Islamiyyah) - Proscribed terrorist groups or organisations - GOV.UK (www.gov.uk).
Sections 11 to 13 of the 2000 Act provide for various offences relating proscribed organisations including supporting such organisations. The penalties can be severe - including in some instances up to 14 years imprisonment.
October 2023 - protest - About 100,000 turn out in London for pro-Palestine rally | Israel-Hamas war | The Guardian
On 27 October, the UN General Assembly called for a humanitarian truce. The UK government abstained. Gaza crisis: General Assembly adopts resolution calling for ‘humanitarian truce’, civilian protection | UN News.
Pro-Palestine rallies aren’t ‘hate marches’ – they’re an expression of solidarity, helplessness and frustration | Nesrine Malik | The Guardian