Well the obvious difference is that in Britain you see police on the streets. In America you call them police but they are soldiers.
Note, US cops are not wearing high viz to be visible.
First off, British police uniforms aren't flashy, either. Second, US police do have high viz cross guard vests and coats.
They wear black like special ops.
Usually only SWAT, K9, and other high risk units that requires a lot of movement wear all black or OD green. The campus police where I work at wear black polo shirts and cargo pants.
They wear combat trousers.
Again, usually, around my area, the only police that wear combat pants are those in special units. Combat pants are light, durable, and comfortable, and inexpensive, especially compared to the polyester pants that is usually issued to police in my area. It's the reason why the campus police I work for wear them. Campus police hardly do anything so it really only make since.
They wear military boots.
Again, comfort and versatility. It really has little to do with trying to be military. Cops have to patrol areas that are less ideal for normal footwear. That's why some cops wear boots.
They carry guns
In a country with the highest private firearm ownership in the world. Also, Most of the rest of the world carry guns too. Britain is just being weird like with their driving. Who ever heard of driving on the left side of the road?
and wear military helmets.
No US cop wears a helmet throughout their shift. Those things are heavy.
They throw grenades.
First off, it's usually special units like SWAT that have access to any sort of grenades. Second, US police don't use lethal grenades to my knowledge. They only use flash bangs and tear gas.
That's a soldier, not a policeman.
That's your opinion, not fact.
The point of police and why they were invented (in England first in the world) ... was because people didn't like having soldiers in their villages. Soldiers tend to shoot people and make people nervous. And so the police was born and 'policing by consent' became a thing. Walking in to town with an army is not policing by consent. It is occupation.
Again, your opinion about US police.