It is not "illegal" to put up an electric fence, temporary or permanent without the approved warning signs, but
a smart-arsed lawyer could take civil action against you if a person suffered injury as a result of them getting zapped.
There's stuff on the net - usually on fence manufactures sites that highlight types of notice, spacing, distance from overhead power lines etc.
I've an extensive electric fence system that feeds the fences around the house (got fed up with cattle leaning over to munch my bushes etc.) and surrounding farm fences. A long run is alongside a public footpath, so I had to be careful. (Takes all the fun out of it!!)