Why would it be an own goal?
The point of the protest is to cause disruption by direct action and bring attention to the issues that XR want to highlight. Outside the bank of England is a pretty good place to do that.
Disruption to the day to day is the goal because XR is trying to highlight that normal service can't be resumed. So if some public transport gets diverted and causes a traffic jam, bringing attention to the issues to many people and the authorities, that's not an own goal, that's the whole point of it.