What do you mean by 'on the sensor'? It's the white line that counts. If you crossed it when the light was red, you've committed an offence and, if detected, you'll receive a fixed penalty notice. (Indeed, unless it was unsafe for you to stop, you'll have committed an offence by crossing the white line when the light was amber).
If your car was over the stop line then you could still get a ticket. My former housemate got 3 points and a NIP through the post which said he had gone through a red light at zero miles per hour :(