//We can lie down on a busy road , disrupt traffic and it’s not illegal . !!//
It is illegal, anne, on a number of counts. The argument would be whether it is lawful (different entirely). The police cannot determine that as their function is the prevention and detection of crimes (which are being committed by the protesters). So they should arrest the perpetrators, pass their evidence to the CPS who should decide on whether court action should be taken. This would include assessing any potential defence that might be forthcoming.