The Black Arrow was a British small-sized rocket, used to launch Small Payloads into space. It was cancelled in favor of US rockets after its 4th mission which was successful.
It is, however, not as powerful in real life like in the game
The correct assembly for a Black Arrow rocket is as follows:
4. Any small payload
The only medium payload you can add at the rocket is the Atmospheric Probe, but it will explode in the air as a fairing can't be fitted.