Measuring instruments have to give the correct readings. If a petrol pump says it has dispensed 50 litres of petrol we want to be certain that 50 litres have actually been dispensed. If a French car manufacturer asks for a 6 mm diameter bolt the supplier in Korea has to deliver a bolt with that precise diameter. A calibration laboratory establishes whether the value that a measuring device gives is correct in relation to the international unit of measure. This is important for the entire production chain. The work of calibration laboratories is therefore the basis for other disciplines.
The RvA accredits calibration laboratories on the basis of the ISO/IEC 17025 standard. Accredited laboratories are obliged to participate in proficiency testing: testing whereby the test results from various laboratories are compared against each other.
You can use the search engine or consult the list of accredited bodies to find details of all calibration laboratories accredited by the RvA. Policy document BR010 describes the various fields of work relating to calibration.