Apologies if someone has posted this already, I'm just catching up on the thread, but I can't see it. I am fairly certain (it's hard to pin down) his is the relevant exemptions to the legislation in question, taking 15/09/20 as the latest version:
The Health Protection (Coronavirus, Restrictions) (No. 2) (England) Regulations 2020
F28(1) During the emergency period, no person may participate in a gathering which consists of more than six people unless—
..........
3.(i)the gathering is for the purposes of protest and—
(i)it has been organised by a business, a charitable, benevolent or philanthropic institution, a public body, or a political body, and
(ii)the gathering organiser complies with paragraph (5G),
.......
(5G) The gathering organiser or manager (as the case may be) complies with this paragraph if, in relation to the relevant gathering, they—
(a)have carried out a risk assessment which would satisfy the requirements of regulation 3 of the Management of Health and Safety at Work Regulations 1999, whether or not the gathering organiser or manager is subject to those Regulations, and
(b)have taken all reasonable measures to limit the risk of transmission of the coronavirus, taking into account—
(i)the risk assessment carried out under sub-paragraph (a), and
(ii)any guidance issued by the government which is relevant to the gathering.