Basically I want to be able to run my 2.1 powered computer speakers and sub from my HiFi using the HiFi as the output but at the same time being able to play music from the HiFi speakers AND the 2.1 system.

The 2.1 system connects via a 3.5 jack which obviously when plugged into the headphones socket turns off the HiFi speakers. Is it possible to stop, turn off this function or connect the 2.1 system via the same speaker output as the HiFi?

If it helps the Hi-fi is A PHILIPS WAC3500D Streamium Wireless Music Centre and the 2.1 system is also made by philip

when you plug your 3.5 plug into the jack, it is pushing a small strip of metal that disconnects your speakers.
You need to circumvent that "interrupt" in the input jack.
probably best accomplished by jumping over it with a small wire (soldered) .

But be aware that headphone-out is not exactly line-out.

Your input to your 2.1 system may not be optimal if you use the headphone out jack.