I can see it now.
this type of valve does not have much adjustment for depth. I have fitted quite a few. Forget about it having to be behind the plasterboard. Just make sure its right for the tiles. This WILL mean moving the mixer forwards.
The cylinder section of the valve. The plate once pushed back in the picture shows 15-20mm still exposed.
7mm tile must be a small-ish format. Ceramic, maybe porcelain.
Looks like its a purpose made stud to house the BIV. So being a small area. . . . . . 6mm backer board, 7mm tile and a compressed bed of approx 3mm. I think theyll get it in.