DP, I certainly hope you're correct and that the IAA is indeed a lot quicker (and cheaper) than the CAA.
I've been trying to get an IMC rating added to my UK-issued JAR-FCL PPL for well over six months now. The average e-mail response time of the CAA is 40 days (based on a sample of two queries, and only after a reminder e-mail sent after 30 days on each occasion). Furthermore, everything I do seems to trigger something else at this stage, and everything costs money too.
(The IMC rating addition triggers a conversion to an EASA PPL, which triggers the need to transfer my medical from the Dutch to the UK authorities and triggers the requirement to add the aerobatics rating. So far I'm up to 7 different forms and 275 UKP in costs - and counting.)