Patsy,
You will need a class 1 medical certificate to train for your commercial pilots licence and a class 2 for your PPL.
For the class 1 you HAVE to go to Gatwick - this costs £400 plus your airfare etc.
The class 2 you can have done by a local AME in the Belfast area - this will cost you about £150.
If you want to be a pilot commercially it is recommended that you get the class 1 done first because you can then have the renewals done with your AME in Belfast at alot less cost. Another advantage of this is that you will benifit from the CAAs reduction in eyesight and hearing requirements after the initial examination. This is good because if you have the class 1 done at a young age the chances are higher in passing and the subsequent examinations by the AME will be easier to pass as you get older rarther than going for the initial class 1 at an older age.
In summary: get a class 1 done and keep the renewals up by your AME and DO NOT let it lapse. Financially and medically this is very good advice for you.
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And up we go!