Rocks or concrete laid on tarmac are not difficult to push out of the way with a vehicle at slow speed if you have a vehicle you don’t mind denting a bit. To stop vehicles you need to link concrete blocks together or dig the rocks in to the surface. Is your plan to limit the areas that can be accessed once trespassers get on site?
I have previously understood that cutting locks to gain entry constituted sufficient damage to invoke s61, obviously the police disagree and s61 seems to give them a lot of discretion, it just says they ‘may’ direct them to leave. See also police guidance here.