If the legal width of the path isnt known (check with council as you wouldnt want to fence and then have to take it down as if a legal width is defined that overrides any other rules) then absolute minimum seems to be about 1-1.2m but recommended is 2m (which would also help avoid moans as with fencing both sides, 1-1.2m would be harder for those coming in opposite directions to pass). Thats assuming it is purely a footpath and not anything higher like bridleway.
Any electric fence involved and you would def want it 2m wide or more and also there is a legal obligation to have signs advising of the electric fence every so many metres.