I don't understand this resistance to turning off track. You are going to have more traffic following your exact track directly aligned below you than crossing you if you turn off to avoid the traffic below you, then a short time later turn parallel to either head back or continue, or divert. You are asking for trouble blundering straight down through everybodies airways tracks, hence the importance of the ICAO requirement to turn off track before commencing descent if able.

So what is the decision? To 'ignore' the ICAO requirement 'because I know better'?

NATS or EURO control.
 

Turning off track/airway is the best way to go, as all the procedures state but my resistance to it (in busy controlled airspace) without communication with ATC, is that nowadays you spend large amounts of time on Radar Headings or indeed direct routings that aren't "on airway."
So i'd love to hear an opinion from NATS or EURO control.

Nopac Etops
 

Descent procedures are spelled out in the Jepps regarding cabin pressurization failure on NOPAC routes. An offset left/right 25 NM to clear the airway, THEN descent to a lower altitude.

For those about to say 'one must descend right away', the answer is no. The procedure is takes into consideration passenger oxygen availability. In my case, 22 minutes on the 777 for the passengers. The lateral offset and descent will not use up the 22 minute availability.

Safe flying gang.

Wholeheartedly agree. Because of the nature of the beast (emergency descent and a fair amount of adrelenin) especially in IMC, it is all too easy to overbank and overcontrol while dropping the nose for rapid descent especially with flight spoilers deployed. A straight ahead descent does not require negative "G" at all - rather like a TCAS RA descend command. In terms of dark night IMC on instruments, straight ahead may be the lesser of two evils.

I think that is quite understandable. Why not give it a second tought.
1) The procedure I quote is ICAO "differencies to Europe". While I agree that is is a great shame that ICAO documents are not available as public domain, the sad status quo is that I cannot give you a link. Yet, the quote says as a controlled flight. In other parts of the document, the word Eurocontrol is quoted numerous times. I get the feeling, that whoever designed this procedure (if able!) and codified it, had a very good European ATC input.
2) The headings you mention, as I see it, serve to make the full use of available airspace by reducing separation to the approved standard of 5 NM. TCAS threshold is 1,1 NM /35 s for RA whichever comes sooner (now I've found it). 420 kt TAS is 7 NM/min.
Let 5 NM separation be the "paperwork" threshold.
Let 1,1 NM / 35s be the "scary-hairy RA" threshold.
Let 0 NM be the ultimate threshold.
If you are kept on a heading to achieve separation 5 NM to a conflicting (same level) traffic, in a 90° scenario it would take you 42 second to hit it. This may very well be the the start of your Emergency Descent, so the everage RoD is about 2000 fpm in the first minute. Target ACAS RA miss vertical distance (now I've found this as well) is 600 ft. By the time you reach the ultimate threshold - 42 seconds, you would have achieved more than twice (1200') the ACAS life guard separation.
Discussion regarding the other two threshold is, I'd say, superfluous. Needless to say, ATC will definitely NOT be happy about an ED. Yet, neither them nor us are payed to feel so, as opposed to finding and exploiting the safest course of action, retaining as much margin as operationally available.
There is a procedure. It may not be the optimal thing to do ad-hoc, but it will save our (a-word) and cover our (b-word). A procedure must be simple, short, unific, and uniqe. At the end of the day, it is the pilot's call.
Cheers,
FD
(the un-real)

Some interesting points lurking from a deeper layer of thought, which aircraft are you referring to?

Use of autopilot is encouraged and a spin of the heading pointer is a simple way to achieve a controlled banked entry.