Any question answered...

Is there an ISO or British Standard (if that still exists!) that governs the accuracy or guarantees the accuracy of weighing equipment?

What are you weighing? There are regulations and inspections if you sell things by weight, to ensure that your scales are maintained at the required accuracy. As above, NPL hold the national standard measures, and your calibration certificate should ultimately be traceable to them or the equivalent authority in your locality. Any weighscale is only as good as its last calibration, so fussy weight weenies regularly recalibrate their scales using standard weights, which for normal purposes need only be accurate to about 100ppm, so they are not hideously expensive, although electronic scales are now so cheap that a calibration weight usually costs more that the scale it is used for.