I have passed my 7th diabetes anniversary and had 2 blood checks a year since then. I have always fasted - water only - for 12 to 14 hours beforehand.
Originally we were told to fast for cholesterol/lipid tests, but in its wisdom NICE advised this was not necessary a few years ago. The reason being purely logistics. Fasting tests were always carried out early morning, which made waiting lists for the tests longer or required extra nurses. However, to get a more realistic triglyceride number and LDL, it is necessary to fast. These levels are likely to be raised if you have recently eaten as the lipids will be swimming about in the blood stream doing their job. As these levels are important, and may trigger the prescription of statins if the nurse considers them too high, it is essential to fast.
As said earlier, it is not necessary to fast for an HbA1c test, but it will be unusual if that is all you have. I always receive the full works - HbA1c, cholesterol and lipids, liver and kidney functions, and full blood counts.