Found it. Section 7.1.9 - Tolerances:
7.1.9.1 Finished tile Surfaces
The surface should be true such that, when checked with a 2 m straightedge with 3 mm thick feet at each end, the straightedge should not be obstructed by the tiles and no gap should be greater than 6 mm.
There are permissible manufacturing tolerances for ceramic tiles defined in BS EN 14411; certain types of tiles, e.g. extruded or large format, might have permissible surface flatness irregularities that cannot satisfactorily be accommodated within the surface flatness tolerance permitted to the tile installer; this should be taken into account when evaluating the achievable flatness of a wall tiling installation.
NOTE - Where adhesives are used, this degree of accuracy can be achieved only when the background surface is equally true.
7.1.9.2 Across joints
There should be no appreciable difference in level across joints (commonly called "lipping") and the maximum deviation between the tile surfaces either side of the joint, including movement joints should be as follows.
a) Joints less than 6 mm wide, 1 mm.
b) Joints 6 mm or more wide, 2 mm.
