Sorry that this reply is a little bit late but I have some information on this topic that is very recent. I recently (1 month ago) looked into converting my foreign instructor rating to a UK one (I have a UK commercial licence though), and here's the deal:
After November of last year, the rules on conversion changed somewhat thanks to the JAA. Now it is not as simple as just getting a "recommendation", you have to do 15 hours of flying and 30 hours of groundschool. The CAA require you to fill out an assessment form (completely seperate to the one you have undoubtedly filled out to convert the rest of your licences, and another £80). They say that they look at each case individually, but they also said that their hands are now tied by the JAA and the best they can give you is a "half course" which is what I described above, so that is what usually happens. How this is considered a "half course" I am not sure since the groundschool requirement has gone up to 125 hours, but for conversions 30 hours seems to be the norm. Anyway...
So you are looking at about £1500 if you are NVQ registered, otherwise you are looking at slightly more than £2000.
Haven't made up my mind yet if I want to do the conversion or not.
Anybody out there interested in hiring an instructor with 600 hours (900TT) of fixed wing instructional experience (private, commercial and night)? Just wondering if it's worth doing the conversion ...
Lazlo