The external stop tap is often located near the boundary of a premises under a small cover, about the size of a CD case. If you have a water meter, it will generally be located in the same pit or chamber as the meter.
Please note that the external stop tap belongs to your local water supplier and you may need to seek permission to operate the external stop tap even in an emergency. If you turn off the external stop tap in an emergency you may need to seek permission from your water supplier to turn the supply back on. Not all water suppliers will give permission to use the external stop tap and if you operate the external stop tap and damage the stop tap, you may be liable for repairs.
Not all properties will have an outside stop valve fitted and this is quite common in older homes, or if the incoming water supply serves both your home and one or more of your neighbours.
If you're unable to find your external stop tap you should contact your local water supplier.