A paramount Western value is that dissenting voices can heard. This can be accomplished in the UK while also maintaining public order. In the case of Armistice Day weekend, many diverse voices will be heard, but the different events will take place on different days and in different locations. The security situation is well in hand.
The British public has spoken out against what they see as the desecration of a day meant to celebrate fallen British soldiers. Couple that with growing fears of rogue bad actors using the conflict to wreak havoc on society, it would be in the interest of public safety to simply keep Armistice Day as it's traditionally been and push the pro-Palestine march to a less controversial date. Sunak, legal authorities, and the event organizers should move this event and are wrong not to do so.