I'll just quote myself from page 1 of this thread.
To start with the constable could start with simply ask them to move off the centreline using powers of persuasion (a cop can ask you to do pretty much anything they like without needing a legal power as you can choose to comply or refuse)
But if you're looking for offences or legislative powers then you can start with Actions endangering safety of aircraft (under ANO) for one (approach surface infringement which is a safety specification). Constables have power to use reasonable force to prevent the offence (i.e. move the people on)
For anyone between the hedgeline and the field boundary then they're on the highway (as the legal highway includes footpaths and verges) and must obey the directions of a constable to proceed in a particular direction.
And actually the posters further up were right - if a constable feels that they need you to do something then there is usually a law that allows them to do it. If there's a law that allows them to do it then they wont be losing their job.
And to the person above commenting on the overzealous use of S44 terrorism act against photographers - it didnt result in 'hats on no biccies' interviews it actually resulted in Parliament saying "it was too wide ranging" and they revoked it and brought in an amended version which tightened up the circumstances in which it could be used