what is recomended concrete grade (british standard) for an automobile repair shop's floor?

Answer 1

I do not know about British standard but I would poor at least 15cm thick fiber reinforced slab. If no fiber you may want to throw down a 15x15cm (or similar) steel reinforcing mesh.

If you intent to put lifts then you must beef up the floor to support the structure per manufacture’s recommendations.

Good luck

Answer 2

A half an inch of slump and 600mm re-inforcing mesh at 100mm depth on a 450 mm thick slab . Jules Australia.