4.9 is a great reading.
Not sure about the UK, but the guidelines we're given as "ideal" here in Oz are:
Fasting (and before meals) between 4.0 and 6.0
2 hours after meals between 4.0 and 8.0
According to Jenny Ruhl's Blood Sugar 101 site:
"What is an Abnormally Low Blood Sugar?
Blood sugars under 70 mg/dl (3.9 mmol/L) are considered to be hypoglycemic. However, if you are not on insulin or a drug that causes your pancreas to secrete insulin, a blood sugar slightly below this range, while it might be uncomfortable, is not dangerous unless there is evidence that it is continuing to drop.
The dangerous levels of low blood sugar--the hypos that require a visit to the ER--are those in the 40 mg/dl (2.2 mmol/L) range and lower. At those levels unconsciousness and brain damage can occur."