Where in the UK can I buy Foam mounting Squares or some Form of double sided pads suitable to put up my bathroom mirror on bathroom glazed tiles?

Double sided foam mnounting squares should be a common item at a hardware store. Howvever, I do not think this is a safe mounting procedure for a mirror in a damp room. You should consider mirror mounting clips that are screwed into the wall or grout line. This will carry the weight more sfaely than a tape. A glass store can sell you the clips and also a mirror adhesive designed to not react with the back surface of a mirror.

Is this the case even if the mirror is small?

Sorry for the delay, how small, what are the dimensions and weight?