Gaden tap feed?

Under Wras regulations (UK) a double check valve should be fitted to the garden tap feed to prevent backflow of contaminated water into the house fresh water supply. They say fit internaly but no room in pipe under kitchen sink as well as room to resite hole through the wall to outside.
Q: Is there any reason the double check valve cannot be fitted in the outside pipe. The pipe will come out of the wall drop down around 10inches to the tap so ample room to fit double check valve?
All external pipe valve &, tap will be well insulated and boxed in easily removed for maintenance.
Thanks in anticipation.