Earlier quoted context omitted.
This is one way of inconveniencing people. Road closures are another way. Like many protest tactics, it doesn't bring about change, until it does. The Just Stop Oil protesters are using an 'earned media' strategy that has proven successful for decades. Publicity stunts draw attention, and even negative attention (in the short term) often yields dividends over the long term.
Who are they inconveniencing? Who are they making reconsider their stance? What are their explicit demands? A protest needs a target audience, a target and a set of demands. They have none of the above. Occupy Wall Street for example had two of the three, but ultimately failed in most of its goals because it didn't have a concrete demand. And that was an example of a protest on a far larger scale.
They're hoping to force change in government policies. This is obviously not happened yet, though they claim a success in halting the issue of new oil and gas exploration licenses in UK territorial waters.
I think their explicit demands are extremely obvious, right there in the name of the organization 'Just Stop Oil.' A visit to their website (cryptically named https://juststopoil.org) lays out 3 simple demands:
Demand 1: No New Oil and Gas Licences – WON!
Demand 2: Just Stop Oil by 2030.
We need a Fossil Fuel Treaty.
Amazingly, they have a whole page on the website which discusses these in greater detail.In all honesty, I find it hard to believe you are as mystified as your post suggests.