This is basically two questions : can I film on private property? and can I film police?

All the reservations about photographing on private property apply, ie you may do so unless or until the landowner forbids it. Thereafter you commit a civil trespass. You also must be mindful of privacy issues and not photograph anyone where they might have a reasonable expectation of privacy. Please see other parts of this FAQ for more details. Police may well point out that you have no permission from the landowner to film. Unless you do, that's a warning shot. Whilst it isn't their job to uphold the landowner's civil rights, if you persist they'll likely view you as hostile and difficult - and find something else they can charge you with.

In general you have the same legal rights to photograph police as photographing any other citizen. However it often doesn't work out like that because the police can always find a charge that can be applied and may result in the threat of arrest or arrest if you persist. Typically this might be obstruction of a police constable in performance of his duty, but there are other possibilities. This is a risk on the public street for professional photographers carrying press cards. On private property, privacy and/or security ssues may magnify the risk.

From this, for instance snapping away inside a police station or at a crime scene would be highly inadvisable without permission. Having said that, police are routinely filmed by CCTV within shops, whenever they arrest a detained shoplifter, so it's very much a matter of context. Unless you have some overriding public interest reason or just like being locked in small windowless rooms for hours on end, we suggest you ask first.

Yes, but only if it is not prohibited by the landowner. The landowner is within their rights to prohibit photography on their property or to require payment for permission.

If there are 'no photography' signs or terms of admission that forbid photography, you commit a trespass by taking photos, and may be told to leave. If you do not leave immediately reasonable force may be used to eject you. You may also be injuncted or sued for damages in a civil court. However the landowner or staff do not have any legal power to demand handover of film or memory cards, nor to require the deletion of images. 

You must ask the landowner, or their agent or representative. On private land open to the public permission will allow you to photograph people without their individual consent, but note that privacy considerations will still apply. You should not photograph anyone in circumstances where they might normally expect privacy without obtaining their consent.

If you take photos on private land without permission, whether photography is prohibited or not the photographs are your copyright.

There are occasions where if the landowner can demonstrate to the court that your copyright represents a financial loss, eg because he himself sells photos of his property, then the court may intervene and award economic copyright to him.

Not unless you are intending to sell the picture for advertising or marketing use. A model release is a binding contract that agrees to relinquish specific future claims of equity in return for a 'valuable consideration'. This is usually payment, but can be prints or some other agreeable exchange.