In reply to Wiley Coyote:
> Probably not on a public footpath because the right of way is just that, the right to pass and re-pass but not to stop
That's a myth often trotted out by landowners.
You may stop on a public footpath to take a photograph, have a rest by sitting down, admire the view and even have a picnic (which I would interpret to include lighting a Trangia) so long as you do not obstruct the right of way.
You may also pick wild berries (eg blackberries) from the footpath or pick mushrooms on the footpath for your own consumption so long as you do not gather them for the purpose of resale.
I am not certain how the law relates to such activities on access land, but so far as I know there is no specific prohibition.
I suppose it all boils (no pun intended) down to the practicalities involved.
Basically what can a landowner do if they catch you having a brew up on a public footpath or access land? They can ask you to stop. If you don't, realistically what can or would they do? They have to prove that you are committing an offence and even then can you imagine the police reacting to a call from a farmer saying "Someone is making a brew on my land - come and remove them!"