PhilD - The validation certificate you get from the FAA states:
XIII LIMITATIONS
Issued on the basis of and valid only when accompanied by United Kingdom pilot license number(s) xxxxxxx. All limitations and restrictions on the United Kingdom pilot license apply.
That being the case, you cannot apply a FAA IR to a UK licence. You can, however, gain a FAA licence and apply a FAA IR to it and that will get you an IMC rating attached to your UK issued PPL.
Fuji Abound - The french do not need to validate a JAA licence issued in the UK. By definition, a JAA licence issued in the UK is valid throughout the member states of the JAA, without formality. Except, of course, if the training for that licence has been conducted outside of the JAA (e.g. USA) in which case the licence has not been issued in accordance with the requirements of JAR-FCL and any member state (e.g. france) may decide not to recognise it.
If, however, your UK national PPL is validated by the DGAC then the priveleges are as laid down in french law.
englishal - You may think that, I could not possibly comment!