Back to the subject: Would it be an overstatement to say that both ATCos and Flt Crew are well aroused by the risks of traffic conflicts when in the terminal area? IMO this concern gets even higher on the list during GA amidst parallel/multi-RWY ops. Perhaps then the chain of events necessary to achieve this type of mid-air collision should include failures in GA navigational procedure/ SID navigation procedure/ ATC blindness/Comms failure or incorrect freq/ TCAS failure or RA ignorance/ Flt crew Airmanship or SA breakdown and v. bad luck on the mean free path stats front.
Not impossible but maybe less likely than some here seem to contend. If you're really worried I can recommend a pair of danger-sensitive sunglasses which turn opaque just before anything really scary hoves into view!
STF, many of the points about UK infrastructural fragility made by the programme were interesting - and this winter's M11 experience (gritting failures aside) could be the first of some chronic jams. What a shame that the balance of the drama was ditched in favour of exploiting fear amongst the flying public when the real tragedies in such a gridlock scenario would almost certainly be happening on the ground due to the paralysis of the Emergency Services.
As for the diversion question, all Flt crew have to risk-manage on this front. Usually, fuel for a nominated alternate will be carried until the GA. However, when the risk of diversion to that alternate is assessed as extremely low, the diversion fuel may be used under certain conditions to extend the holding time available for the destination airport. Nb that the fuel and the aircraft don't care how you use the fuel as long as you land with what is called 'reserve'. Therefore, you can improve the chances of avoiding diversion and gain time to consider other options such as weather/ATC updates (and the possibility of a diversion closer to the hold as a further fallback option). A real diversion from the GA right on planned fuel is a major concern in a busy ATC environment - especially if 40 other aircraft want to go to the same place. For this reason it would seem prudent to declare an emergency if you are not going to land with reserve. NB reserve fuel is ultimately available for use if a further emergency occurs to delay your final landing by which time we're well into MAYDAY territory and the sort of bad day out only to be experienced in the simulator. UK ATC are outstandingly helpful in these situations - another reason why the programme was in my view defamatory to ATCOs at London - you just don't get any better than these guys and girls...