Tigs2,
Provided Halfords can get the equivalent cambelt, getting the 360 serviced there shouldn't be detrimental to the Ferrari warranty.
However, if I were you, I would stay at the location during the work and insist that you were in the car during the inevitable 'test drive'.
Because of the Halford service engineer mentality, I would rather take mine to a recommended 'back street' garage to get the work done. Mainly because they will have a passion for the brand and treat her with respect.
Every penny saved on the service, means more left over for the fuel tank!!
SS