UK plumbing might as well be on another planet compared to the US.

Your old valve was probably blocked with minerals, so this new one may just be flowing better. Is the pressure in the house greater? It should be the same since I'm assuming you have a gravity feed system.

There should be only one way to fit the valve, and if it's not leaking and the pressure doesn't seem excessive, you may be fine, you're just not used to the noise.

It sounds like the new valve is not opening properly. A partially open valve will tend to vibrate, causing a sound that will reverberate through the pipes.

Bal valves do not have an lose parts tat can vibrate.

They can create flow turbulence when used for throttling flow (partly open to restrict flow) that makes the rest of the piping vibrate.

Ball valves (like gate valves) are not really intended for anything except full on or full off.

From the OP's description, I understood it to be a ballcock valve in a toilet tank, or is it some other tank I'm not seeing?

I think you're right, Randy. It's a float valve, in a header tank. Probably a really old setup.