Hi Feonagh:
The differences won't be as marked as you fear, where ever you go; all will be based on the Eurocontrol 50, so provided you're thinking in the right ballpark, no-one is going to hang you for 10kts, or a couple of thousand feet difference.
However, an appreciation of the reality is what you need to concern yourself with - that's what works in the Verbal Assessments: a small single may have a theoretical ceiling of 12,000ft, that doesn't mean you'll ever see one up there! Think about what's on the flight plan, not what's on the Top Trump cards...
