What exactly is a UK IMC rating?
This is also know in Europe as an Enroute Instrument Rating or EIR, which allows flights under Instrument Flight Rules in the cruise phase, but not for departure or approach. It is based on EASA FCL 825


  FCL.825 En-route Instrument Rating (EIR)
  
  (a) Privileges and conditions.
  
  (1) The privileges of the holder of an en-route instrument rating (EIR) are to
  conduct flights by day under IFR or in IMC in the en-route phase of flight, with
  any aeroplane for which a class or type rating is held.


The curriculum to obtain an EIR is lower than for a full Instrument Rating (IR), especially the number of hours for theoretical and practical instructions have been lowered:


  (2) instrument flight instruction.
  (i) The instrument flight instruction for a single-engine EIR shall include at
  least 15 hours of flight time by reference to instruments.


As GdD has pointed out in his answer, this is not the same as the UK CAA IMC Rating. See his answer for more information on this.