do you need to pass all 14 exams
That depends on what your ultimate goal is.
Don't have the reference in front of me so I'm shooting from the hip here;
You have two options:
Validation requires 1000hrs of Commercial Operations and 500 hrs Multi Crew.
Two written exams if I'm not mistaken, Human Factors and Airlaw.
Downside to validation is thatyou cna only fly the registration of the country that issued the validation. As in only G-reg if it was the UK CAA.
Conversion (more like addition in your case) requires you pass all 14 written exams and a checkride w/examiner. In your case that could probably be done during a type check or type recurrency.
You will then hold a standalone JAA ATP which allows you to fy any reg in JAA-land.
Anybody, feel free to correct if I'm off-base here.