I wish I knew why Manchester was made the controlling agency but it's lost in the mists of time!
I suggest part of the reason is historical. As Spiney or his colleagues may recall, the Manchester Control Zone, including the LLR, used to be Rule 21 CAS. Aircraft within the LLR were therefore
SVFR flights, within 'Manchester's Airspace'. Liverpool, in those days, had a Special Rules Zone, with it's eastern boundary coincident with the western edge of the (original) LLR.