The vehicles that have been cascaded out of Stagecoach London so far are the ones that Macquarie didn't sell and lease back to raise finance (whether it's a case of wouldn't or couldn't is debatable).
It's probably worth noting that (unlike First from what I can tell), Stagecoach London is held/owned/managed separately to UK Bus (their provincial operations), so any potential vehicle cascade is considered in financial terms against a sale to an outside party. (In essence, a provincial Stagecoach company looking for a cascade is told "here's the price the dealers are offering - equal or better it"). There's no "forcing" of vehicles onto operations - but they may well be told "ex-London (or indeed any specific vehicles) are all that's available - take it or leave it.
The Cumbrian Tridents you refer to are a bit of a red herring. Those that kept breaking and being generally useless were the S-BWC batch which, while being ex-London, were cascaded out a decade ago. I'd say it's unfair to attribute their poor performance to their life in London, and would firmly suggest that it is down to specification (3-speed gearboxes and low speed differentials are not conducive to sustained high-speed running) and age.
That said, the more recent London cascades to Cumbria (now open toppers) have proved troublesome, though this is more a simple case of reliability.
