One piece of advice is to actually put what you want to talk about in your subject heading.
Of those three countries the UK has by far the most sizeable flying training industry, and is probably cheaper than Greece, if perhaps not Bulgaria. It is the obvious place to go of that list.
The UK has many good schools, some of them affordable. Get hold of the Flyer Magazine "Learn to Fly Guide", which is as good a guide as exists. If you're travelling internationally and want a recommendation however, try Teeside in Dundee who are well run, and set up for people doing intensive courses.
G