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Initial Class 1 (Professional Pilot) Medical Examination | Medical | Personal Licences and Training
General recommendations are to get it sooner rather than later just in case they find something which excludes you from a career as a pilot (and so you don't waste your money hour building).
However, as Mark1 says, if you ask them, then most AMEs can give you an idea if you are likely to have anything to worry about (mine said no...
). If you don't have any known conditions, then you should be fine. If you do, then you'll need to do some research and bring documentation with you to Gatwick.
Oh - and if you are worried about something specific, rumours are that they are very approachable on the phone.