DPE's have to abide by the Practical Test Standards - it's the same throughout FAA land.
That is very accurate. It won't matter where you do your oral - at busy commercial airports like Oakland or "Slippery Rock Municipal", it'll take 2-3 hours and it is thorough. You don't have to get every answer right, but they do ascertain whether you know and understand regulations and the functioning of your aircraft.
The only advantage I would see for Slippery Rock Municipal is that there are no wait times for departure, virtually unlimited touch and go practice without overly complicated radio work etc...
Not sure about the CAA Medical at Gatwick, but normal JAR Class 2 Medical would preclude monocular vision (which is a shame, because the advantages of distance estimation, often quoted as the main reason for needing binocular vision, do not play at the distances a pilot would focus (e.g. for landing). The only excuse I can think of is that, when I get a spec of dust or some suntan in one eye, I still have the other to see where I'm flying). The medical board of the Belgian CAA would refuse you the class 2 medical.
Congrats on starting the effort
. Are you sure there are no FAA flight instructors in Ireland ?
Good luck !