Forgive my ignorance, but why, in non terrain critical situations, do we not descend to FL100
Because not all aircraft are the same, not all FCOMs ( Flying manuals in old money) are the same, and not all flights are fuel planned in the same way. ( and jet aircraft burn more fuel per mile the lower they fly)
One example - Mid-Atlantic on some aircraft on some routes, if you unthinkingly blast straight down to FL100 it is quite conceivable you will then have insufficient fuel to reach any of your nominated alternates....in fact on a bad day you might not have enough fuel to make any land at all...that's why some airlines/some manufacturers FCOM's state that in the absence of significant terrain always halt the decent at an intermediate level, in our case FL150, (the level used for the planning of a decompressed diversion), and only continue down to FL 100 when landing is assured. You're not going to kill passengers at a cabin alt of 15000, seeing as they're seated and not required to perform any exacting mental tasks then the majority will OK. As a result the cabin crew can begin selectively taking passengers off the masks, saving the gas for the minority who may need it
In a real situation over London, I think i'd want to descend to either 090 or 080 (depending on direction) as it means I won't smash into something else! Or am I missing something here..
Yep, I think you have. If you do descend to FL80 or FL90 you'd better hope you miss all the traffic in the OCK/BNN/LAM and BIG holds, all FL 70
and upwards, plus the stuff heading into the holds and all the stuff climbing out of the various London Airports.
Personally in a "real situation" over London ( in fact just about anywhere these days, even the ocean) there's so much metal flying around I might take at least a few seconds, having got crew and passengers onto Oxygen, to coordinate and/or at least declare a Mayday before initiating any descent at all.