They had cargo ties to hold their placards during transit. Of no practical use for locking on.
The metropolitan police has since says it 'regrets they were unable to join other protestors'.
Between this and the online safety bill there are some dreadful dacronian laws making it onto the statue books in the UK in recent months. Compounding other forms of monitoring and criminalising speech under hate and anti terror laws.
It definitely could happen here.