They measured Thack Moor because the height according to OS maps was 609m - just short of the magic mountain mark.
BBC: Ordnance Survey confirms mountain status for Thack Moor
2.
Thack Moor in the North Pennines, about 12 miles north of Penrith, is 2cm above the 609.6m (2, 000ft) mark that signifies mountain status by Ordnance Survey's (OS) definition.
BBC: Ordnance Survey confirms mountain status for Thack Moor