IRT
Many of the posts have good points - and some questionable ones.
Remember that the privileges of the IR (Multi) permit you to command a multi engine aircraft, on a Public Transport Flight, under Instrument Flight Rules, to single pilot minima, in all classes of airspace, and anywhere in the World. (subject to specific exceptions)
The IRT is possibly the only skill test you will take with a CAA Staff Examiner - just about everything else is taken with an Authorised Examiner.
The IRT Examiner is testing to a Standard clearly defined in CAA FCL Standards Doc 1, Vs 3 (should be available off the web, but it is 38 pages)
The weather minima is defined in para 3.4, and it is the applicants responsibility to say if the flight goes ahead, although the Examiner may decline to continue with the test even though the applicant is willing, if he/she thinks that the weather (especially wind/turbulence) would prevent an accurate assessment of the pilots ability to be made.
In para 4.1.3 it states that a failure will not necessarily result if a flight limit/boundary is exeeded - the examiner has discretion.
In para 4.1.4 it states that the examiner will make allowance for adverse weather conditions. Conversly, you might assume that an Examiner is also capable of making allowances for excellent weather conditions in which case his/her tolerances might be somewhat less flexible.
If you are unable to cope with busy RT in controlled UK airspace, in genuine IFR conditions, then really you are not ready for the IRT.
My humble advice would be to make sure that you train on a de-iced aircraft, and experience real icing conditions. Ensure that your training includes as much real flight time, in busy airspace, as is possible in genuine IFR conditions (Winter is ideal especially if you are training from a "proper" airport, where you might additionally get some night IR hours in your logbook as well) Make sure that your training includes just about every type of procedural approach you can think of - the examiner can request more or less anything, and some of them do - this is not a driving test.
Although its not universally true, the person who has done most of their IR training in a sim, is the person usually looking for the nicest and quietest day on which to do the IRT, on a route second guessed from the examiner, and which has been practised a dozen times in the sim.
Real flying in IFR conditions isn't like that.
Good luck, and I hope my comments are useful.