You will need an approved harness, window net, extinguisher, battery isolator with remote pull wire so it can be operated from outside the car,
race suit, plus your car will need to log booked with cams, plus a few other things CAMS website has a detailed list of requirements for circuit racing. For a good look in you could have a go at regularity, usually run by the MGCC at Symmons and the requirements are only same as running a club day I believe.