There are two different scales for measuring blood sugar levels mg/dl and mmol/L. The first one is used primarily in America but Britain uses the second.
According to Mayo clinic you will be diagnosed as pre diabetic if your fasting levels are between 100 and 125 mg/dl. To convert these numbers to mmol/L you divide them by 18. So in Britain, possibly 5.5 to 6.9. mmol/L. I have not checked the actual figures used in Britain but they must be somewhere near these.
edited after checking some numbers.