The Water Regulations Advisory Scheme (WRAS) is a conformance mark that demonstrates that an item complies with high standards set out by water regulations promulgated in 1999 in the United Kingdom.
The WRAS term has become a 'shorthand' within the UK covering three separate areas.
The first is the WRAS company itself which is a subscription membership company. The subscribers of WRAS are the 26 UK Water Suppliers.
The second is used as shorthand for 'meeting the Water Regulations of 1999' (which is not in fact the case).
Thirdly, it is used for approved plumbing fittings.
It is enforced by the UK water supply companies. It covers all plumbing systems, water fittings, and equipment supplied, or to be supplied, with water from the public water supply. It is not illegal to install a product such as water fittings in the UK without the WRAS mark but you may have to prove compliance in other ways. It is legal to design, manufacture, import or trade in non-approved items. It is legal to use non-approved fittings but only in certain specific areas, typically where human health could not be compromised.
The Water Regulations 1999 are enforced as stated above by the UK water supply companies (who essentially act as appointed/authorised enforcement for the UK Government) and this is the fundamental legal requirement. The WRAS approval number is a way of proving that the item meets PART of the above legislation. So a plumbing fitting might be tested and approved by WRAS - but if installed and/or operated incorrectly might breach the Water Regulations 1999.