It's not even a case of needing a retro-fit as HUD was an option in other countries at launch of the series IV. Not sure why Lexus GB left it out, they even blogged about it as a feature before the vehicle was released - I guess it was to hit a desired price point or to hold things back to make the facelift more attractive in 3 years time 😞
I wouldn't say it is an essential feature but I find it very useful and would probably pay extra to have it on my next vehicle. It's the same with many features, once you have it, it is then difficult to go without.