My GP's practise triggers at +7 mmol/L fasting (two tests to diagnose).
7.8 mmol/L is the figure quoted on this site (http://www.diabetes.co.uk/diabetes_care/blood-sugar-level-ranges.html).
Actually, that figure isn't a figure for diagnosing, or fasting. It is just a figure they show that a non-diabetic should be at after eating. The two fasting tests over 7 you quote is pretty standard although you can be pre-diabetic below that. Most surgeries now seem to do an HbA1c test to confirm diagnosis which looks at the effect of your average blood glucose levels over the last month or so.