I have told to fast completely, apart from plain water, for any cholesterol tests but it's ok for other tests I believe. Your doc or practice nurse will advise you on your specific test.
I'd have thought it was safest only to have water.
I'd hope that your blood test appointment would be timed for early in the day, so it shouldn't be too much of an imposition.
I have to go for a blood test and avoid any stimulants prior to the test so no caffeine, alcohol or tobacco. Annoyingly, I cannot get any medic to agree on for how long before hand I need to avoid these products, nor to absolutely agree on what should be avoided as one added cheese to the list!