Strip the driven plate out and measure the thickness.
Nothing wrong with the clutches on our cars but the slave cylinders are woeful - cheap tat engineered down to a price.
Wouldn't be a problem as the OP stated if they left them external to the gearbox like on the Citroen, as it takes all of 10 minutes to change them.
Did a clutch on an Astra a few years back and the quality of the replacement was noticeable compared to our cars.