SENSOR/SWITCH THAT TURNS ON EXT HDD'S & NAS WHEN PC IS POWERED ON

APC has some UPS units (I have the 750W model) that will control some of its outlets based on whether or not the device plugged into a Master outlet is powered on (and using more than just standby juice). This might work, although these outlets are NOT battery-backed, so if power fails, they will shut off even though the computer will keep running.
An alternative, if speed is not required, is use 2.5" drives in USB enclosures. Many of these need no external power to operate.

Get a 12 Volt relay (Sounds like a trip to the auto store, or home depot) that is low current (coil) and contacts are rated at 120 AC (yea 240 in Merry England) and current of say 1 to 2 amps.

Tap off the 12 V (yellow and black molex) and run to relay coil). Cut power strip cord and attach side to plug to one set of Contacts and power cord wires to stip to the other contacts. (Only need to break Hot side to run thru contacts). Cheap and simple About $5 ->$10 and about 20 min.

Added
If you could get a 5 Volt relay (6 volt might work) you could do the same thing, but using a USB port/cable and using the % V output on the USB port - Just make sure the current required for the relay coil is LESS than the max current available from the USB port.