In the UK the vast majority of scales in check-in areas are owned and maintained by the airport company.
The operator of the scales - the airline or handling agent - has a responsibility to ensure that the owner keeps the scales in working order and accurate. They can do this by audit or by seeing the check certificates from the airport. Surprisingly, it is only because of the monetary transactions (excess baggage charges) that result from using the scales that require their checking and not the safety of the aircraft.
If you want to challenge the weights ask the handling company AND BAA for a copy of the validation of weight certificate.