The information you have regarding FAA to JAA is not correct. The CAA will NOT take into consideration FAA medicals. There was once a proposal to swap FAA CPL with 500+ hours for JAA medical and licence, but this never came about.
It also depends what you failed your class 1 on, will you get an FAA Class 2?
All be it, don't take mine or any one else from the forums word for it. Contact our friends at Gatwick with your CAA ref and as them the question, you will then get the answer in writing from them.
I hope you can get your problems sorted out. I also have a restricted CAA Class 1 because of colour vision, but have unrestricted FAA medical, so I know your pain. All the best mate and never give up.
Also if you plan to take an FAA CPL you will need to pass the FAA written paper. This can be done in the UK by Flight Saftey at Farborough and Willow Air in Southend. Remember the FAA writtens are a totally different affair to the CAA ones.