Hiya Vik.
Shame about the job you've had done, it really isn't good, and needs retiling, it's scratched, wonky, poorly grouted and generally not very good, but:
You did employ a builder, and not a tiler. BIG difference.
Mosaics are hard enough to fix on a perfect flat wall with nothing in the way. Kitchen walls are full of sockets, notoriously bumpy and generally awkward for mosaics. Also these mosaics unless you have quality tiling tools are difficult/impossible to cut, the metal mosaics especially. Black grout is vile to use and takes time and experience to get good with, plus as a builder i doubt he would even have known about fine particle grouts.
Horses for courses here i'm afraid and to get a quality job from these mosaics means employing a quality time served tiler, not a builder.
I can understand you are disappointed, but i would be too if i was the builder and my customer text me with a complaint, i'm sorry but a phone call is much better. Then to bombard him with British
Standards this early would certainly have annoyed him.
I would say try to phone him and see if you can reach an understanding between you and your builder.
Tiling is a skilled trade and it would be nice if the public understood it for being one rather than getting a builder in and then asking our advice once the job is ruined.