In my experience marketed usage time on UPS is a fairy tale. As a rule of thumb I drop 50% off and may be approaching reality.
UPS market in the UK right now is horrible, I originally thought it was part of the global silicon crisis, but people have been posting on TPU in america they can buy a 900w cyberpower pure sinewave for $150, so it seems either the UK retailers are scalping or the UK has a supply problem.
On the battery life. So 600w capacity, 17 mins run time at just under 25% load, max charge.
In my experience the higher the load the less efficient (probably at very low loads as well like craig siad), and also run time at below 50% charge is much worse than above 50% charge. The run time seems lower than usual, but is ok if its accurate.
Does it have a usb port to allow monitoring via software? Thats my only issue with this model you got as otherwise for UK pricing the value is good.