what i decided to do was all my flying from my first ppl hour to when i finish my IR in the uk. it is more expencive but in the end atleast for me i think its the better choice as the airspace is alot more restricted here.
if you intend to finaly get into a jet job you must compleate your IR in a multi engine aircraft within the 3 years of the ATPLs (the CPL must also be compleated 3 years from finishing your ATPLs). single engine IRs are not very useful for jets.
if you have enough money, i suggest you do your ppl (if you do it here you will be able to fly cross channel in a G reg aircraft) so you can fly once every couple of weeks or somthing and build up your hours that way. what i did was as soon as i finished my ppl i went into my atpls and did some flying during the weekends to keep myself current. i then finished of my hours building (you CANNOT start your cpl without 150 TOTAL flown hours) and went direct into my CPL. when it comes to licence issue you will need 100 hours PIC though.
if money is a bit tight for you right now what you can do is do your ppl then just buy the books for your atpls (which do cost a bit) but dont join to a course. that way you can study for your atpls then when you are ready, join a distance learning course for the brush ups and sit the exams. dont forget though the whole atpl course is soon to change to EASA system so no one knows how much change is expected.
as for moving from country to country i know the uk licence is widely accepted all over for a direct convert but not the other way around. its best you do your reasurch before putting your money down anywhere as only you know what you plan to do and where you want to work.
as for your 1st class medical, if you intend to do your cpl here for a uk issue one you will need a uk 1st class medical. bear in mind each country has different standards for passing...