Y'all:
I lived in the UK for a couple of years and shot HFT at the Bisley Field Target Club just southwest of London. Scott's comment about "guessing" isn't too far from wrong but the guesses are very educated.
The typical strategy is to set the scope up focused at 25 to 30 yards depending on preference. Shooters then learn to guess distance based on how far out of focus a target is, Also; kill zone size at various distances is very specific and can be used to get a general yardage. Bracketing is also used extensively just as we do in the US version of the game. For example; a 1.5" kill zone is 3 MOA at 50 yards and 16 power, the same kill zone is just under 3 MOA at 55 yards.
There are entire websites on the subject if you want to more than what I listed.
Cheers,
Art "Ponda" Womack