You will not have a problem with adding an IAA rating to your CAA licence.
You can add a rating from any JAA member state to your licence, no problem...but let's say you have a CAA PPL and you want to get your CPL..it has to be done in the UK or in a CAA approved school abroad.
Can't offer you any information regarding the training at Weston...I asked a question regarding training in Ireland a few months ago and got no information out of it!
best of luck with your training
