YES
The answer is yes, Uk , along with other JAA countries are applying properly JAR's, and as it is writen on JAR-FCL 1 any JAA CPL licence holder having an FI rating valid and with a valid medical can work in UK with no need to do any paperwork or conversion of any kind. I did myself work in UK and had no problems at all.
As far as i know almost all countries that are on the JAA are applying them except FRANCE and NORWAY, were they require you to have your licence convalidated by them.
As Lomcovak say, France wont recognize your FI rating, but with some pestering and patiente it can be done, i got the recognition of my FI rating in France, took 7 months and had to do a flight test with an Examiner-Inspector of the french DGAC to get it (it was the first case of a foreign FI rating being validated).
By the way, Locomvak,if you are interested send me a PM, and i'll tell you more about the validation of your FI rating.
The French DGAC tries to protect their market (FTO's are not doing so well over there) by doing this ilegal things (remember not only they aren't appying fairly JAR's , but they are also not applying EU rules that say titles and licences have to be recognized troughout the EU cou ntries to favorize the free movement f labourforce etc etc.....)
It's not a nationality problem, even French nationals with foreign JAA licences are having trouble in their own country.
The other JAA countries are OK, i know of foreign JAA licenced pilots working in Portugal, Spain ,UK, Italy, Germany, Malta etc etc.
Hopefully EASA will take over licensing in the JAA countries soonish and we'll get rid of all this nonsense.