What it means is:
1. Keep your SP IR(A) valid and you don't need an IMC Rating.
2. If you want an IMC rating on your licence, you'll have to pay. Why would you want one? To maintain IMC rating privileges should you let your IR lapse for any reason. In which case it'll remain valid until 25 months after the date on which the IR was issued, re-validated or renewed.
Then there's confusion in LASORS yet again!
E3.4 An (IMC Rating) applicant who has held an ICAO or military green Instrument Rating or a UK professional pilot licence aeroplanes in the 2 years preceding the date of application for the IMC Rating will be exempt from taking the initial IMC Flight Test and written examination.
seems to indicate that putting an IMC rating on a UK-issued JAR-FCL CPL/ATPL is purely an administrative action - no need for exam or test if you've held a ICAO IR in the previous 2 years. So, it you're a retired airline pilot who last passed a multi-pilot IR re-validation 18 months ago, you could apply for an IMC rating which would be valid for a further (25 - 18) = 7 months.
Why then, does LASORS go on to state:
The holder of a JAR-FCL professional aeroplane licence with a valid multi-pilot IR(A) is not automatically entitled to fly using the privileges of an IMC Rating in single-pilot aeroplanes. They will be required to obtain a separate IMC Rating endorsement on the basis of their most recent multi pilot IR proficiency check or skill test. In this case, the IMC Rating will be valid for a period of 25 months from the date of the IR(A) skill test and will not need to be revalidated if the IR remains valid. If the IR(A) lapses an IMC re-validation flight test is required.
This indicates that, obviously you must have an IMC rating to exercise such privileges in a SP aeroplane - but the last sentence seems to be nonsense as it contradicts the 'held an IR in the last 2 years' right. I guess what they mean is that if your IR lapses, you'll eventually need to re-validate or renew your IMC rating like any normal PPL holder would, i.e. by IMC rating test. But any remaining IMC rating validity period between the expiry date of the IR and the expiry date of the IMC rating must surely still apply? Otherwise they'd be stealing up to 13 months of IMC rating time off you! Perhaps it should read "If the IMC rating lapses an IMC re-validation flight test is required."