The GT Line S would be an amazing car, I’ve sat in one and it eclipses the GT Line and that stereo upgrade is much better than a Bose one. That car below £5,000 would be tempting, below £4K would be awesome.
We can live in hope.
GT Line S has uses Meridian 14-speaker Sound System compared to Bose 8 speaker system in the i6 Ultimate. Bose for example is famous for emphasizing the mid-range, so will depend on ones ears which system is best. However many in car premium sound systems are licensed out to third party manufacturers.