The Water Supply (Water Fittings) Regulations 1999 in England and Wales, and the Water Supply (Water Fittings) (Scotland) Byelaws 2014 are UK national requirements for the design, installation and maintenance of plumbing systems, water fittings and water-using appliances. 

With respect to fittings such as valves these regulations require that: 
4.(1) Every water fitting shall be of an appropriate quality and standard and be suitable for the circumstances for which it is used. 
4.(2) For the purposes of the regulation, a water fitting is of an appropriate quality or standard only if:  

A) It bears an appropriate UKCA or CE mark 
B) It conforms to an appropriate harmonised standard,  
C) It conforms an appropriate British Standard 
D) It conforms to a specification approved by the regulator.  

Currently three approval/certification schemes operate in the UK for Regulation 4 compliance in relation to water valves and other fittings: 

NSFreg4 
WRAS 
KUKreg4 

All three of these schemes provide the legally required testing and certification demanded by Reg 4, and therefore offer the exact same levels of product approval. 

For reasons associated with the history of the UK water industry WRAS has been seen as the default certification required to demonstrate compliance with the water regulations, this has led to oem designers and other valve specifiers insisting on the need for WRAS approval for fittings connected to the water supply.  This is now unduly restrictive, WRAS is now one of the three certification schemes available in the UK any of which can offer the same level of compliance certification with Reg 4, suppliers are under no obligation to be WRAS approved.  Compliance with the water regulations is the key requirement, ahead of any particular approval/certification body being used to certify that compliance.  

The enforcement of the Water Regulations is a statutory responsibility of the water authorities, and they employ water regulation inspectors to attend site and examine water systems to ensure compliance. Water authorities are now stating that products must be compliant with Water Regulation 4, not simply WRAS approved.