Two things.
First: I personally think it is not worth replacing a few cracked tiles. It will only be temporary based on the quality of the installation. Correct me if I'm wrong but those joints look about 10mm wide? No tiler I know would ever use a joint that large on pressed glazed ceramic wall tiles, no matter how sizey they may be, so a further guess is that this may have been a DIY project or a builder?
The way the grout joints are also cracking and caving in would suggest there is movement somewhere, causing both the tiles and grout to crack. No support beneath the grout may also indicate they have been spot fixed or there is insufficient coverage of adhesive, which is another major factor in cracking.
Second: Unfortunately I don't recognise the tile. It's not one from any of their mainstream ranges so most likely an exclusive to either B&Q or Wickes. Johnsons City touchstone tiles are the same size but design and colours are very different, but again, there may still be other exclusive ranges available in the "sheds" that could work. One last option you have is to find the closest match tile in a larger size and cut it down to do a patch-up.
