Hi Simmo, what sort of a reading is it? I think for HbA1c the US is the only place different from the rest of the world. For a US meter reading you divide the level by 18 to get the UK equivalent.
Just for info, HbA1cs are reported similarly all over the world... though that is about to change!
In the UK, most Commonwealth countries and some Scandinavian Countries blood glucose levels and other blood tests are reported in Si units. For blood glucose this is mmol/l.
In the rest of the world, including much of Europe they use conventional units. For blood glucose this is usually mg/dl (but sometimes as here in France in mg/l, an important difference in where the decimal point goes on any calculation).
The conversion factor depends upon whats being tested, so as Copepod says its vital to know both what the test was for and the units used.
here are some common conversion factors.
To convert mg/dl of blood glucose to mmol/l divide by 18
To convert mg/dl of HDL or LDL cholesterol to mmol/l, divide by 39.
To convert mg/dl of triglycerides to mmol/l, divide by 89.
For other things look here:
http://www.globalrph.com/conv_si.htm