Ouch, proper nightmare situation – we went through the exact same thing when I ordered these handmade Moroccan tiles that were basically all different sizes. The splashback became this proper stress point because every glass cutter kept saying “we’ll make it fit” but the measurements were always off by a few millimetres, which looks absolutely awful behind irregular tiles. My joiner finally pointed us at trade glass supplier Uxbridge after we’d wasted weeks on near-misses, and the difference was night and day because they actually manufacture in-house from their Uxbridge workshop instead of just cutting and hoping. They took proper measurements, sent drawings for approval before cutting, and the precision was spot-on – no “close enough” nonsense. Took their specs back to our tiler and suddenly everything lined up perfectly. If you’re dealing with handmade tiles and irregular measurements, finding someone who actually crafts bespoke solutions with proper quality control makes all the difference between a proper finish and a “we tried” job that drives you mad every time you walk into the kitchen.