Thank you all for your interesting and thorough replies. It has given me plenty of food for thought.
I realise that sooner rather than later the decision to progress to foiling will come but I will still go ahead with a light wind TT for a couple of reasons, one, it will aid progression
now and two, my wife who is finding the whole learning process trickier than I, is unlikely to be ready to touch a foil until she is fully confident on a TT, so it will get good use. In the end, if we have shared quiver comprised of a foil board, a large TT, a standard TT, and a range of kites, I suppose it is conceivable we could both ride at the same time with me on a foil and smaller kite, her on the TT and larger kite.
The Spleene Carbon Pro Session 54 seems to be a popular option but certainly at the 'serious' end of the spectrum. Will a 'beginner' look like a pillock doing the 'walk of shame' with a carbon fibre board like that?!
After that the Flydoor6 sounds like the more ubiquitous option. Less of an investment if (when?) it gets lost and maybe a bit more discrete.
And an interesting nod for the Oxygen V3 for being shorter and therefore maybe more playful, though how much does a short but wide board do for leg burn vs a longer thinner one or is it a subtle difference?
Also, thanks for the thumbs up for the Naish Boxer. My logic there is I want a kite that will get up, stay up, and relaunch in light winds and in beginner's hands. From what I've seen and read the Boxers will do that - just a shame they're ugly!
Finally, the suggestion of longer lines sounds sensible. With regards to that, how practical are line extensions? Is it realistic to have one bar with lines at 15-20m (for the strong wind days) with a series of extensions of say 5m and 10m for when they are needed for the light wind, or does each extension add some 'inaccuracy' to the tuning and handling of the kite because of subtle differences in line lengths and therefore multiple bars and fixed-length lines are better?