As a radar controller, you can’t assume aircraft will adjust their speed based off the aircraft around them, when they are being provided radar services.
6.14.7 — It is important to note that if you have only given a speed assignment to a single aircraft (when multiple aircraft are involved), it is unlikely this will provide the necessary separation […]
The pilot should always be aware of their surroundings but it’s our job as controllers servicing the pilots to help them avoid conflicts. Here’s a few examples in the manual.
In addition, “Adjust Speed to Follow Aircraft Ahead” can be useful but does not replace the necessity for specific speed commands, therefore Controllers should use it sparingly. As well as this, if used without assigning a specific speed command to the aircraft ahead, it is likely that the lead aircraft will start to dictate the entire flow into the airport, Controllers must avoid this.
You technically can use “Adjust Speed to Follow Aircraft Ahead” but specific speed commands will always be more effective. You have complete control over the separation with no guesswork.