A fairly mild warning / experience.
Tiling bathroom with "cheap" tiles. 200 x 250 flat white. 6mm thick,
25p each I think. Glaze is fine, they look OK, so why spend more ? Only
thing, they are slightly bowed variably in both dimensions, something I
never though to check. At most about 1.5 mm in the centre. I had a
look at some more expensive (and thicker) tiles, which were perfectly
1.5mm doesn't sound like much, but I was trying to get a solid bed
around the shower, and after lifting one to check, I noticed the middle
was not in contact. In the end I buttered the back of the tiles and
wiped it off flat, which solved the laying problems. But it did slow
things down a lot as you can imagine.