It is one of our basic rights to be able to gather and make use of free speech. We have a right to gather and a right to free speech by law.
With the excuse of Covid legislation, the government and the police are imposing restrictions on gatherings. Protests are however exempt in all three tiers. You can read it in the government document, page 24: https://www.legislation.gov.uk/uksi/2020/1374/pdfs/uksi_20201374_en.pdf