For the early battles the this might have been part of the problem:
"Twenty-five thousand soldiers had gone to South Africa with the new Lee Enfield, the mechanics of which were the same as the superseded Lee-Metford except for the grooves of the rifling. The variation caused the weapon to shoot 18 inches to the right at 500 yards. Fortunately, the error was discovered during the last days of 1899 and the superintendent of ordnance factories in Britain immediately proceeded to make a back-sight that would rectify the blunder - whether or not it restored the British soldier's confidence in his marksmanship is not mentioned." Hill of Squandered Valour: The Battle for Spion Kop, 1900 by Ron Lock pg 87.
Boer vs British tactics meant that all to often the British soldier was in great numbers out in the open and was being shot at by a Boer hidden in a concealed position. But yes, all too often British marksmanship was poor.
Cheers,
Mark