The water regs state ......
Water supply systems shall be capable of being drained down and be fitted with an adequate number of service valves and drain taps so as to minimise the discharge of water when water fittings are maintained or replaced.
the guidance notes state....
Inlets to all float operated valves, cisterns, clothes washing machines, dish washing machines, water heaters, water softeners, and other similar appliances should be provided with service valves to facilitate maintenance.
It is generally considered good practice, on new installs, to put service valves in line with taps. Though by these guidance notes not a requirement.