JAR-FCL1.175 says that you cannot act as a pilot under IFR unless you hold an Instrument Rating , or are under training or doing a skill test.
which is paragraph (a) and it then goes on to say:
(b) In JAA Member States where national legislation requires flight in accordance with IFR under specified circumstances (e.g. at night), the holder of a pilot licence may fly under IFR, provided that pilot holds a qualification appropriate to the circumstances, airspace and flight conditions in which the flight is conducted. National qualifications permitting pilots to fly in accordance with IFR other than in VMC without being the holder of a valid IR(A) shall be restricted to use of the airspace of the State of licence issue only.
My reading of that, with Schedule 8, suggests that a holder of a JAR-FCL licence issued by the UK may fly under IFR in the UK.