We aren't taxiing out single engine yet! - but we are txiing in single eng if taxi time over 4min'ish.
Have done a fair bit of 'push and parking' recently to ensure on-time push mainly due to our ground operators dissapearing if we have incurred a slot. When doing this we have shut-down both engines at the hold using only the APU. Care must be taken to ensure ALL checklist items are complete, otherwise you could miss something.
I would imagine that sooner or later we will adopt a single eng taxi out policy as in the US, just a matter of time IMHO.