The decision to use such a modular assembly technique has proven its worth already, with AT200 units being assembled at two plants globally, and AT300 units being assembled at three plants globally.
Hitachi could have done everything at Newton Aycliffe, including building a friction stir welding shop to make the bodyshells, that's something they're still considering, but the entry into service date for IEP in particular would be several years later. Kasado is well equipped and experienced in testing new designs, reviewing CAD/CAM issues and ensuring the production units don't suffer any of the snagging issues encountered on the pre-production units.
Newton Aycliffe could have done that too, but it would have taken time to build up those skills, to build and equip testing workshops and laboratories, and to have enough pre-production units passing through to actually practice on, and time wasn't on the TOC and ROSCO's side here, with the acute shortage of capacity and impending disability regulations coming.