Good advice 'thing' on the nav exam. But all of the exams can be taken before you fly if you want to. They'll probably be more difficult, but they are possible unless you
know the theory. And again the exams aren't difficult.
On the medical side of things, an ENG1 merchant navy thingy is like a grade 4 or 5 RAF medical thingy... they count for diddly squat. You probably have a good idea about your medical fitness, and that is what the AME will approve to make it CAA official. If it is a worry do it straight away.
The 'average' PPL students solo at about 10 - 15 hours. A medical is needed for this (some schools require an Air Law exam pass too
). If you think you will pass the medical then start training, if not don't and do the medical first. But it really isn't that strict. But but but at £100+ per hour be wise.
As a guide for exam reading, I'd start with Air Law - APM book 2 (as above) & Met fact sheets (available from met office website), as well as Meteorology - APM 2 also. To make the grade as quickly as possible, all I'll suggests is that you visit as many places/read as much as possible,
and understand it.
If you can courteously drive a car you can fly an aeroplane...
The only thing to remember is that you need air flowing over the wings whatever the situation