I'm not sure what question you are answering. If its 'how would you run a traditional large change programme' then yes what you have documented is pretty reasonable. The development of a target operating model 'up front' is crucial as you need to know what the fuck it is you are attempting to deliver.
If the question is, how would you of used Programme management disciplines to deliver Brexit, then thats a different question. What was needed up front to develop a TOM would have been difficult and protracted. A better way of doing it would have been to form a small, senior, empowered, cross party steering group to consider the options and whip their respective parties into line. Perhaps with some discussion, some elements of a solution could have been found to appease Scotland and built a better solution to NI but who knows?
I cant help feeling that whatever the option put forward, it would have been too aligned to the EU for half the stakeholders and not aligned enough for the other half. A year or even 18 months of painful option wrangling and trying to get as many on side as possible would have made everything that followed a great deal easier.
I have significant doubts though that May/Corbyn would have been capable of leading such an activity, particularly considering the right wing of her party and various terrorists. After all, if you remember there was a process through HOC to vote on options and look how that turned out. So to make it work and achieve an operating model that the country could get behind required a very different type of politics than the incumbents were capable of providing leadership for.
In you response to Vic, you made all of that sound straightforward but I think it was anything but and explains why so much has been kicked down the road. Thoughts?
Once established though the workstreams are a great deal easier as they have a good handle on scope and what it is they are setting out to achieve.
One element which has been particularly badly done is readiness testing. In the past I have set up a matrix of workstreams on programmes both 'vertical' ones with their own scope of work and then 'horizontal' ones that offer a 'shared service' to each of the vertical workstreams. Typically this would include business readiness testing which would ensure all vertical workstreams are independently assessed before they can be given a go decision.
Big subject but all theoretical and wont change an atom.