I would have thought questions like "which countries would recognise my British degree as a suitable prerequisite for a PhD?", "how would I secure funding?", "which institution has a good name for research in my subject area?", "what differences are there between the British PhD and ones in foreign countries?", and "could I put up with the cultural differences?" for example would be questions in the forefront of you mind rather than focussing on finding "the best unis in Europe".