It sounds like there is no easy answer that I can point to and say "aha - that fits my needs".
The Aegis is expensive and relies on the NCE, which is not that user friendly for kids - but does allow them to be mobile around the layout. I also already have the NCE but the Aegis adds the cost of a whole another control system so no real cost-advantage to going this route for me.
The black Z21 relies on a smart phone, which to me is not ideal (I do prefer a physical control knob).
The ESU system looks great but if not available as a UK version I'm reluctant - it looks like it's been 6 or 7 years in the running so it rather looks like it's never coming.
All of these are an investment of £500 or so, give or take - I can understand why so many go with the Hornby HM7000: Perhaps I should just either sell the Powercab and go with that or else buy the HM7040 Legacy Dongle must be plugged into the RJ12 socket of the Powercab panel and be done with it - Then the ability to move around the layout is then covered via a smart phone, whereas the thumbwheel can be retained via the Powercab's throttle.
Has anyone done this who could share their experiences?