There are no easy answers. I contacted one of the UK’s specialists in rights-of-way law last year (he literally wrote the book), and he was not willing to give an answer other than “it depends”.
But … assuming you’re talking about England and Wales (it’s different in Scotland):
A right-of-way (and also a public highway) is essentially a defense against being accused of trespass. You have the right to pass along it, and to stop, and case law suggests that there are few things you can’t do while stopped, so long as they are not breaking any other laws. So: you can stop for a picnic, or to take photographs. You can’t obstruct the path or highway while stopped.
If you were walking along a right-of-way on National Trust property, their bye-laws still apply.
“Rights of way” and “public highway” can be designated either due to historic use, or under statute law, but the law around their use seems to mainly arise from case law e.g. DPP v Jones:
I conclude therefore the law to be that the public highway is a public place which the public may enjoy for any reasonable purpose, provided the activity in question does not amount to a public or private nuisance and does not obstruct the highway by unreasonably impeding the primary right of the public to pass and repass
Rights for what you can do on Open Access land are more closely defined in section 2 of the Countryside and Rights of Way Act 2000:
Any person is entitled by virtue of this subsection to enter and remain on any access land for the purposes of open-air recreation, if and so long as—
(a) he does so without breaking or damaging any wall, fence, hedge, stile or gate, and
(b) he observes the general restrictions in Schedule 2 and any other restrictions imposed in relation to the land under Chapter II.
I think flying a drone is “open-air recreation”, on the basis that standing around using any camera is.
Chapter II allows all sorts of restrictions to be placed on the use of the land by relevant owners or authorities, e.g. to protect flora, fauna etc.
Schedule 2 is a long list of incredibly evil things like carrying metal detectors, carrying a cat, swimming in a river, picking a flower etc (no seriously, they’re all covered, look it up). The three that seem most relevant to me are:
intentionally or recklessly takes, kills, injures or disturbs any animal, bird or fish,
without reasonable excuse, does anything which (whether or not intended by him to have the effect mentioned in paragraph (q)) disturbs, annoys or obstructs any persons engaged in a lawful activity on the land,
engages in any activity which is organised or undertaken (whether by him or another) for any commercial purpose.
Tl;dr:
- I think you can legally fly a drone from a public right of way or from the side of a public highway so long as you don’t obstruct it, and you aren’t breaking any other laws or by-laws or causing a nuisance (in its legal sense, i.e. disturbing landowners in the reasonable use and enjoyment of their land)
- I think you can legally fly a drone on open access land so long as it’s not for commercial reasons and you don’t bother anyone or disturb wildlife or livestock, and you aren’t breaking any other laws or by-laws (e.g. in National Parks)
None of this is simple though. Angela Sydenham’s book on the law around rights-of-way and access to land is 669 pages long and costs £125. You can be confident that none of the landowners, police etc understand it either, and arguing over the details of the law with them is pointless unless you’re planning to hire a real lawyer rather than rely on Random Internet Dude.
I’ve flown from Rights of Way (and other public paths), and from my car parked at the side of the road, and I’d happily fly from access land. But I’d do what I can to keep out of people’s way and give them as little reason as possible to want to report me.