let us have a look at how the BBC is reporting it:

http://www.bbc.co.uk/news

interesting use of the word 'existential' because it's normal context is within philosophy and as an ABSTRACT or CONCEPTUAL adjective or noun...existential...existentialism....

Cameron is using the adjective in a CONCRETE way, here...what he really means to say is that IS is an 'existing threat'...but that doesn't sound 'intellectual enough' does it?

additionally, Cameron...is saying something along the lines of 'it's either them or us'....'the two cannot exist together'....

i.e. he's going for the Armageddon route....the CLASH OF CIVILISATIONS program....no doubt something that akiva had put onto his chip....

"we are quite sure IS is an existing threat - as run by one of Obama's units" replies MI5 ONLINE....and that really complicates the problem, doesn't it?...can we say that in this particular instance - that it is highly likely that IS wasn't involved (in relation to 'ms phoolan devi-bandit queen' but I have no further information about the possible 'others')....unless 'Obama's unit' claims that it was (even if it wasn't)....just how complicated is this going to get?