Yes, have done so. Partly anyway:
Got an FAA PPL, then did Irish groundschool, added British FI in US, added Czech type rating.
Initial Medical + 4 times renewed in 4 different countries, NEVER in a JAR country.
Am currently outside JAR land, a colleague of mine flies a 737 here on a French JAA licence with a non-JAR 737 rating added to it. - Most JAR CAAs will add non - JAR ratings to their JAR licences (with some limitations), I believe the British do not.
I know the CAA LGW will accept all this, as I enquired personally a while ago.
PM me for anything specific. Hope this helps, IP.