Legally, the academy trust is the employer of academy staff, but the amount of delegation varies between academy trusts, so check what will be expected of your governing body in these arrangements.
Existing staff are transferred to the academy trust by the Transfer of Undertakings (Protection of Employment) Regulations 2006 (known as TUPE). Effectively, this secures the terms and conditions of employment for those staff to be no worse than at the point of transfer, although it does not set them in stone forever.
Academies are not bound by the School Teachers’ Pay and Conditions Document or local Green Book arrangements for support staff, other than for those staff transferred via TUPE. Staff appointed after the conversion to academy may be employed under different terms and conditions.