Yay! Finally a thread where I disagree with everyone!
This is, above everything else, a marketing stunt. It is all about showing the new model, and Lexus is far from the first to show a non-performance model premiering at Goodwood.
I think showing it there is a good choice because it shows confidence in the driving dynamics. Lexus would not go near Goodwood with a model that couldn't take the corners properly.
It is important to show the UX as being a worthy successor for the CT. Remember that the CT was initially branded as a better looking Prius with improved interior and driving dynamics at the cost of the higher price, slightly lower fuel efficiency and less versatility. I remember this fondly since I was one of those who traded a Prius for a CT. I know that the UX doesn't outright replace the CT in all markets, but with the CT remaining the same as in 2011, it is a hard sell for anyone who wants to trade an old CT in.
And why would I trade my CT in for a new one? My old one is just as fuel efficient, completely reliable and otherwise the same. If the UX can offer an upscale interior, improved versatility, equal fuel economy and no disadvantage in driving dynamics, then it would be a better replacement candidate. We already know the interior is amazing for its class. The only outstanding question is driving dynamics.