As Homeguard says if you have a UK National CPL or ATPL then you don't need an IMC rating because the privileges are built in. See ANO Schedule 8. If you have a JAA Professional Licence then to exercise the privileges of an IMC rating then it must be entered into Part XII and you must have a certificate of revalidation that is current. Art 31.
Your ATR IR will not have any validity in respect of the IMC rating.