Those straps have never been used for that purpose, the CSPEP has been in use for many years, years before this pack was even designed.
The manufacturors website even states the straps can be an aid to parachuting, which means the pack was designed on a flawed concept
http://www.cqc.co.uk/rucksack90patterninfantry.asp but the straps have been left on the pack design regardless.
There is no way that these straps could be used to secure the pack to the parachute harness, a release mechanism is necessary and the pack was never designed/configured to be used without the CSPEP (which also allows other equipment/weapon systems to be carried and has some other safety-critical features to prevent the load moving when exiting the aircraft).
The CSPEP in the link I posted would have entered service around 2000, as it was trialled in 1999. The previous version had two release 'hooks' and the newer version replaced these hooks with the red handle (as shown) which links to a release mechanism made of kevlar loops. But there was always a CSPEP, it is not possible to make a safe parachute descent (with equipment) without one!