OK, it doesn't need to be specifically WRAS approved no but it does have to be tested and certified (approved) by one of the 3 UK recognised testing bodies - NSFreg4, WRAS and KUKreg4, so that it complies with Reg4 of the Water Fitting Regs (Scot bylaws) 1999. Only once it has that can it then be used on UK water supplies. Any fitting not passed by one of these bodies, if following the regs, should not be used.
WRAS of course is currently the most well known and anything used in a domestic setting is more liable to have been passed by WRAS as complying with Reg4, than the other 2