I'm surprised that no one has quoted the original text:
1.175(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.
The UK is clearly a "JAA Member States where national legislation requires flight in accordance with IFR under specified circumstances (e.g. at night)". Therefore, within the UK, "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."
Surely that permits a UK PPL without instrument qualification to fly IFR in circumstances that meet the restrictions of paragraph 3 of Schedule 8 Section 2 Sub-Section 1, which Jim59 quoted above.