The Highways Agency which is a Government body that builds and maintains footpaths has issued guidelines. The desirable width should be 2000mm . 1500 MM is acceptable where 2000mm cannot be achieved due to obstacles. 1000mm is the absolute minimum.
The guidelines of various local authorities who have responsibility for some paths in their area use the same formula.
The formula may not be legally binding but would be highly persuasive to a judge and there would have to be a very good reason why any path is less than that recommended minimum.
I hope this helps.
If there are any further points please reply I will be happy to respond
The situation for a private garden would mirror that of the guidelines but we need to remember , public or private, people are so wide. There has to be an element of common sense.