Much easier to move them under their own power to Birkenhead, then undertake component removal there to keep others going. I believe 507/8 maintenance is done at North these days? Also easier as its a straight forward drag up the Borderlands Line to Wrexham and straight down via Shrewsbury and the Marches direct to Newport. No need for reversing then as far as I know! (Except on 140s drag which for some reason reversed at Shrewsbury to go to Crewe for the night, then head back via Shrewsbury the next morning)