My 13-pin is also under the rear valance when stowed but can't be checked in the stowed position as the cover flap won't open.My 13 pin sits under the rear valance and was checked. The Government website does not exclude a check on the 13 pin if it should be “ hidden”.
https://www.gov.uk/government/publications/car-parts-checked-at-an-mot/car-parts-checked-at-an-mot