Wherever they go, you'll need to have access to them for maintenance purposes so don't lose them in walls.
Typically they would be fitted on bath/shower mixer supplies since there is an obvious risk of backsyphonage of bath water through the shower hose.
Also if a mixer valve takes water from tank fed hot and mains cold. It may well be that the water regulations require more extensive use, but I have to admit to not knowing chapter and verse on this.
Outside tap wants double, most outside tap kits incorporate one. You can buy a "shower check" which fits in line with the shower hose. Quite hard to get hold of, one of the catalogues does them, don't remember if it's bes or screwfix. My local plumb centre had some before the good people left and numpties moved in. Now they look at me like I'm an alien when I ask for them.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local