I can't see any reason why photography would be treated any differently with any other "normal activity" that footpath users would do.
For example is it permitted to use a mobile phone whilst on a public footpath? I can't see anything in the legislation about mobile phone use.
Would using a pair of binoculars breach the rules?
How about using an mp3 player to listen to music as you walk?
a mobile phone would come under usual accompaniements , as would an MP3.
Binocs/ cameras don't however because the sailent difference is that you don't use them as you are walking, you walk to a point and then stop to use them - as soon as you stop you are no longer freely passing and repassing but instead on the land engaging in another activity which is technically trespass.
of course 99.9% of the time no one gives a good **** - the trespass through non permitted activity is most usually implemented against people cycling or horse riding on footpaths , but the 0.1% of the time is where you get a landowner that doesnt like the activity, and has a perfect right to prohibit it on their land
(the other common falacy is that a public right of way is public land - it isnt it is private land to which you have a right of access for a specific activity)
THe other thing to bear in mind is that you only have a right to freely pass and repass on the definitive line of the path (which is often not where the path actually is on the ground), and only within the corridor of the path (usually 3ft width for footpath or 8ft for a bridleway - but not always definitive widths vary )
and lastly you don't have a right to obstruct the passage of other path users , so setting up a tripod or even standing still for a long period of time could be a no no (again mostly no one cares, but ocassionally you get a landowner who does)
(incidentally I'm not relying on interweb/wiki armchair expertise here - I was a rights of way officer for 5 years, and I've acted as an expert witness in trespass cases - Just to stress though everything I'm saying here is under English or welsh law - scotland is different)