It is not in the water regulations.
You have to have a service valve on the hot water supply. For cold supply, you only have to have service valves on anything with a float (aka ballcock) valve WC cistern central heating header tank, cold water tank etc.
It is generally considered good practice to fit an isolation valve e.g 'ballofix', to taps, shower valves and the like, because it makes it easier for maintenance or replacement. Instead of isolating the whole water supply, you can simply turn off the valve to that appliance. For the sake of a little over £1 each, I am appalled the house builder's plumbing contractor did not fit them.
Then again I suppose they thought it is two more areas for a potential leak!
As for where they are located if fitted:
My first guess would be in a cupboard such as the Airing Cupboard (they should be labelled!), in the floor void below the valve, or within the stud wall cavity behind the valve. In the last two cases an access panel should have been provided.
It is not a huge problem to turn off the water supply stopcocks and change the shower valve though.
It should have lasted for more than 4 years too. Who was the house builder?