Depends on what kind of IR you have or had and if it's still valid. Probably too late now I know, but this would have been better sorted before you exchanged licences
LASORS Section E3.4
JAR-FCL Professional Aeroplane Licence Holders
JAR-FCL professional aeroplane licences do not have in-built IMC privileges. The holder of a JAR-FCL professional aeroplane licence who wish to obtain an IMC Rating must complete the requirements specified below:
2. JAR-FCL CPL(A) or ATPL(A) holder with valid Single-Pilot IR(A)
The holder of a UK issued JAR-FCL professional aeroplane licence with a valid single-pilot IR(A) does not require a separate IMC Rating endorsement as the privileges are included within the IR(A) privileges. If, however, you wish to obtain an IMC Rating you will need to apply to PLD for endorsement. The normal IMC rating issue fee will apply unless application is made in conjunction with an application for the initial grant of an Instrument Rating (IR).
3. JAR-FCL CPL(A) or ATPL(A) holder with valid Multi-Pilot IR(A) only
The holder of a JAR-FCL professional aeroplane licence with a valid multi-pilot IR(A) only is not automatically entitled to fly using the privileges of an IMC Rating in single-pilot aeroplanes. They will be required to pass an IMC Flight Test and apply to PLD for a separate IMC Rating endorsement.
What you may be thinking about are holders of pre-JAA, UK national CPLs and ATPLs, which had (and still have) embedded IMCr privileges
If you still want to get an IMCr;
Exemptions and Credits
• An applicant who has held an ICAO IR(A)...or a UK professional pilots licence aeroplanes at some time in the 10 years before the date of application for the IMC Rating will be exempt from the requirement to undergo a formal course of flight or ground training but will need to pass the ground examination and initial IMC Rating flight test.
• An applicant who has held an ICAO IR(A)...or a UK professional pilot licence aeroplanes...at some time in the 5 years before the date of application for the IMC Rating will be exempt from taking the ground examination
• An applicant who holds an ICAO IR(A)...and has passed a single-pilot IR test in the 24 months preceding the date of application for the IMC Rating will be exempt from taking the initial IMC Flight Test and written examination. Applicants will be required to apply for the issue of the IMC rating within this period, and the IMC granted will be valid for 25 months from the date the last IR test was passed.
I'd guess for you, that you're exempt the course and ground exam, so just the flight test (plus voluntary revision) and then application to Gatwick. Well worth doing I'd say and if you don't do it soon, you might not be able to.