I put one UK base under my preferences, the others in Europe (can't actually remember exactly what I put, it was six months ago now).
No idea if it makes any difference to whether you get called for assessment or not. I felt that by putting several options that were all quite far apart it would show a willingness to be flexible. In all honesty though I don't think they really look at that stuff and the subject did not come up at all during phone call or assessment day.