If you don't do what the police ask you to, you will not be able to run your event, it's as simple as that. This is often a financial issue for example. :
I could name several smaller independent festivals in the UK that have been made financially infeasible by large imposed policing costs and had to be subsequently cancelled. Many would argue that often the policing costs are un-warranted and that the police may use this as leverage to prevent events that they don't approve of but no one has demonstrated this successfully to date. Certainly however it's true that there is wild variation in the level to which different events are burdened by the police but of course this can also be attributed to other factors such as the quality of their pre-event planning / paperwork and the differences in approaches that different regional forces take.
Personally however, as both and event professional and an engineer, I'd like to know whether this was imposed on Download as a condition of their license, and if so what their justification for requiring it was. Secondarily I'd like to know how it is that they have been allowed to do this on the quiet. Certainly I believe that storing images of peoples faces would be covered under the data protection act.