Strap yourself in.... The standards are fairly loose on grout joint requirements for walls but a little more detailed on floors.
In BS5385 part 1 (wall tiling) sub 7.1.4, the joints should be "of uniform width, true to a line, continuous and without steps. Allowance should be made for an adequate width of joint". - to make sure you have the right allowance, the tiles should be checked, loose laid if required to gauge what size joint would be needed to overcome potential issues with stepping and wonky grout lines.
In BS5385 part 3 (floor tiling) sub 7.1.3.2 (straightness of joints) it makes it a little clearer that the tile dimension may not allow perfectly square joints:
"The joints between tiles are an important feature of any tiling installation, particularly when small tiles, wide joints, or contrasting coloured grouts are specified. The width of the joints between tiles should be even and of regular dimension **subject to the manufacturing tolerance for the type of tile specified**."
"Generally. tile joints should be straight in alignment unless they are, by design, irregular in shape. Special attention should be paid to large areas of floor tiling where the joint can be sighted"
**Which is BSEN14411, size tolerances for the actual tiles are +/- o.5% from the average to a maximum of 2mm and the same for rectangularity. If the tiles are out by that much, its likely your grout joints will follow suit without significant adjustment
5385 is a code of practice, not law of the land so you just need to show that you have attempted to do the best you can. If the tiles are within tolerance but still so crap that you couldn't achieve the required finish, prove that you've done your best with what they gave you by showing the size variation on a couple of loose tiles.
However it would have been much easier for you to get paid if the expectations of what could be achieved were set with the customer before you started. Probably needed to spend a little more time and attention to detail on setting out and checking the tiles before diving in with the adhesive and pointing out potential problems to the customer.