As you state that you're a 'QFI', presumably you have a current military IR? If so, you can apply to the CAA for an IMC rating with little further formality, but you will still need to pay the issue fee.
Or did you actually mean that you're a civil FI(A)? If so, do you hold a UK CPL or a JAR-FCL CPL? The UK licence incorporates 'automatic' IMC and Night, the JAR-FCL CPL does not.
Are you sure that Cabair didn't actually mean that you had to have the 'no applied instrument' restriction taken off your FI(A) rating? If so, you'll have to do the training and test as specified in LASORS H1.4. If you have a JAR-FCL CPL and haven't yet got an IMC rating, you'll have to do that first (up to 5 hours training - as you'll already have 10 hours IF from your CPL course - plus a 1.5 hour test, there'll be both the Examiner's fee and CAA issue fee to pay for in addition...) When you've obtained your IMC rating, you'll then have to complete at least 5 hours dual IF training and 35 hours theoretical training all of which must be with an FIC instructor qualified to conduct instrument training at an approved FIC FTO, followed by the relevant elements of a FI Skill Test in a single pilot SEP aeroplane with a suitably qualified FIE(A).
To add MEP privileges to your FI(A) rating, first you need to hold a MEP Class Rating. (Or ME Type Rating). If you haven't already got one, that'll mean not less than 6 hours flight training plus 7 hours theoretical ground training, a theoretical knoledge examination and a Licensing Skill Test, all to be completed at an approved ME FTO. Plus the Examiner's fee and CAA issue fee, of course. Then, having obtained your MEP Class Rating, you'll have to build up at least 30 hours P1C on the applicable type or class of aeroplane, 10 of which has to be in the year before the date of application. Then at least 5 hours of flight training in a MEP aeroplane, plus 25 hours of theoretical training, all to be conducted by a FiC instructor approved to instruct in the aeropalne at an approved FTO - plus the relevant elements of a FI Skill Test with a suitably qualified FIE(A). Plus the Examiner's fee and CAA issue fee......
Sorry to sound so gloomy, but if you only hold a JAR-FCL CPL(A) with ATPL theoretical knowledge credit ("fATPL"), don't forget that you must complete your IR(A), not just IMC Rating, within 36 months of passing the ATPL(A) exams or ther credit will be lost. (See LASORS D1.3).....
I guess you're going to need a water-cooled credit card!