Kazza - Think you may be right and nobody currently makes 150x200mm tiles (6x8 inch). You may be able to find a compromise here: Wall Tiles & Floor Tiles - Walls and Floors
Alternative options - buy larger tile and cut to size; remove all tiles and re-tile; get a company to make your tiles to your spec.; check out a few local reclamation yards.
they are a nightmare to find i had a job a few months ago and had to tile a previous feature wall with matching tiles to 150 x 200mm tiles,i ended up matching up height wise(different length) with different tiles and keeping it a feature wall