For anyone who cares, its all about covering ones posterior and no-one wanting to take responsibility for anything less than what BS recommend. Chances are going for smaller grout joints will not be a problem providing the preparation is perfect and you have the appropriate movement joints and/or decoupling in place.
But, adhesive/grout manufacturers will give a condition along the lines of "products are guaranteed on the strict condition that the relevant standards for tile fixing are followed (BS-5385)". Gives them a quick get-out if the defecation hits the oscillation. Then its down to the tile manufacturer to whether they will guarantee their tile if fitted with a smaller grout joint than what BS recommend.
Minimum recommendations:
BS5385 Part 1 - Internal wall tiling (7.2.1.6):
1-2 mm
BS5385 Part 2 - External wall tiling (6.3.3):
Extruded tiles = 5-10mm
Dust Pressed (ceramic/porcelain) = 3mm
Natural Stone calibrated = 3mm
Natural Stone un-calibrated = 5mm
BS5385 Part 3 - Internal and External Floor tiling (7.1.5):
minimum 3mm
All the above dependant on tile dimension irregularities.
Parts 4 and 5 are additional standards based on different service conditions and natural stone types. No specific joint widths given so revert back to the underlying parts 1, 2 or 3 which should always be followed.
