"Dear Ed,
Going on your description of events, I would certainly make a complaint if it was me. Did you take a note of the details of the police officers concerned? And do you have any witnesses to them demanding you delete the photos or theatening smash your camera?
There is no law that gives the police the power to deliberately damage your property, nor to demand that you delete images or to delete them yourself. If you were suspected of being involved in a crime (i.e. the fight) and your images might represent evidence of this, there is an argument that they might be within their rights to treat the camera/memory card as evidence. But, as you will realise, evidence is supposed to be preserved, not destroyed!
You may have heard of the new s.58A of the Terrorism Act, which makes it an offence to elicit information about, among others, police officers, where this is of a kind likely to be useful to someone committing or preparing an act of terrorism. Some people have interpreted this as making it illegal to take photos of the police, and there may be some police officers who tend to take that view, but it does not make all photographs of police an offence (though it does complicate things for photographers). Even so, even if the police thought you had committed this offence, the correct course would have been to arrest you and take your camera into evidence (since the images would be the evidence of the offence). Deleting or demanding you delete your images would not be within their power.
I hear of a lot of cases where photographers have been told to delete their images by the police, or even by security guards. It is not just rural police, sadly.
I dare say you have found this already, but just in case you haven't, here is an online form and an alternative phone number for Avon and Somerset Police Complaints:
https://www.avonandsomerset.police.uk/contact/general_enquiries/complaint_form.aspx
I hope this helps, and I would be most interested to know how you get on.
Linda"