I accelerated to overtake a lorry pulling over onto the roadside and exceeded the 30mph limit in doing so and was caught at 39mph on a mobile camera van (obscured from view by the lorry)
I’m afraid you will not have a defence in relation to this matter. There is no defence to speeding on the basis that you did it only to overtake. You would also struggle to defend the matter on the basis that the signage was obscured.
If the signs were correct then you will still be convicted if you accept the speed alleged. The only defence would be to show that you were not exceeding the speed limit.
Have you got any points already? If so, how many?