BC's are only accurate when you do the work and calculate them for your gun, load, and shooting conditions. It would be surprising if your data in the UK matched that of Grand Island, Nebraska. You can email Hornady and ask what altitude and velocity they used for calculating their BCs...~Muir
Hornady say in their reloading manual (6th edition, Vol 2, page 7):
All ballistic coefficients and ballistics tables in this book have been adjusted to standard conditions.
Altitude = sea level
Temperature = 59 degrees F
Atmospheric pressure = 29.53" of Hg
Relative humidity = 78%
These are the standard conditions for the [US Army's Prooving Ground at the] Aberdeen test site.
Hornady also say that their BCs are relative to the G1 model standard projectile. If somone is using a different standard projectile model, the numbers they obtain will look different. Hornady also says that their BCs are calculated from test firings undertaken in their 200 yards undergound range.
Often best to get it from the horse's mouth.
-JMS