When you are filming police try not to talk to them. Avoid:

"Is it OK if I film?" (That isn't up to them.)
"Sorry officer." (You have done nothing wrong don't apologize.)

Try things like.
"I am filming from a safe distance."

If they start yelling at you focus on:

"Am I free to leave?"

Every time I've said this the officer has responded with "shut up" (ok??)
I'm happy to comply with *that* one and keep filming from a safe distance.

"Am I free to leave?" *will* mark you out as an "annoying do-gooder who doesn't like police"

But this could be an improvement on seeming like someone they could trick or trip up more easily.

Prepare to be ignored. Police will try to provoke you so they can arrest you or stop you from filming. Police will hit you unprovoked or try to knock your phone away.

Don't waste time "reasoning" with them. Just get the video so more people can see what they are doing.

Practice how you will film and what you will say. It will be a very stressful situation and it's tempting to just try to "be reasonable and talk" you aren't dealing with someone who is reasonable. They want you to stop filming. They may be very angry and emotional and lash out without warning.

This can be shocking if you watch too many TV cop dramas.

You are our eyes and ears. The things you film could vanish forever so don't get drawn in to a back and forth.

If you are in a public space and "a safe distance away" and "not getting in the way" you are following all of the laws when filming openly.

Shockingly even if you are clearly doing all of these things a police officer may react as if you aren't. Be ready for that reaction.

The more people do this the more video there will be so the truth gets out to the public. But, even if the video isn't important it sends a message about your community and expectations.