My understanding is that the policy doesn't prohibit all BBC staff from participating in political events and demos as partisans and campaigning for their fave causes on social media. The prohibition is against publicly-funded BBC journalists who cover political, national and special interest affairs participating in partisan events and making social media posts expressing opinions one way or the other about the topics, causes and orgs they're supposed to be covering with a degree of impartiality and evenhandedness.

This used to be standard practice in journalism. And it seems a reasonable policy for the BBC, which is funded by license fees coerced/strong-armed from the British public, and is supposed to portray the broad swathe of viewpoints found among the diverse British population. Instead, the BBC today employs only/mainly educated, well-off elites who come from the top tiers of British society and live in London; and much of the journalism done by the BBC nowadays represents an extremely narrow band of British viewpoints, meaning that of "woke" SJWs with misogynistic views.