It will all depend on what your MOT requirements are, but not with standing that seatbelt mounting all comes down to the overall strength of the mounting point usually measured in kN of force. (i.e. the amount of force able to be applied to the mount before breakage) I would refer to the MSA or FIA rule books to see what the requirements are in a race vehicle as these are usually very good and beyond the needs of most. I know when we were racing back in NZ the seatbelt anchors had to be either the factory ones or using approved eyebolts or Grade 8 bolts (certain size, M12 comes to mind) through the chassis/body with a large 3mm steel washer on the reverse side, if I remember correctly something in the region of 100mm2.
