While a contract exists between MoD and the contractor, most designs are "Under Ministry Control" (as opposed to Under Contractor Control).
Sorry, but I find it rather hard to believe that BAE could bounce this one back to the MoD.
"Under Ministry Control" or not, cannot the buyer at least assume that the supplier is capable, as the other posters here have suggested, of meeting the most rudimentary of airframe fabrication skills? Or is it assumed that MoD must inspect and endorse every stage of the manufacturing process?
Should this actually be the case I would say there is something very wrong with the UK defence procurement process.