After reading this I don't know which is worse, the thieving scum or the insurance companies (oh, hang on, they're one and the same). I can't believe you can buy a SS gold lock only for it to be 'downgraded' later, either!
Fortunately I don't have to leave my bikes in a vulnerable area. Both of my 'proper' bikes are too posh to count under household insurance, but my commuter is not much for anyone to get too excited about nicking anyway. Trouble is, half the time if they can't nick it they just kick in the wheels out of general yoof yobbery. My good bikes are in the garage, which has extra security on the door, and then they are ground anchored to a motorbike-level anchor that would need a Kango to get out, using some pretty heavy duty motorbike locks. None of these is portable enough to actually carry, and some of the locks weigh as much as the bikes! Even so, the motorbike mags did similar tests to the ones in Allen-uk's PDFs and came to the same conclusion: even the best lock only lasts a few minutes against someone who's properly armed and the lighter a lock is, generally the weaker.
All in all I tend to agree that all you're really doing is complying with t+cs, and hoping that the evil human excrement thief will just pick the next bike along if it looks easier.