It will very much depend upon the exact beam configuration whether they are UK legal. Some US lights are not asymetric as in having the left hand edge of the beam projected into the hedge with the right edge cut off. They would likely be OK, but provide very poor road illumination with just a flat beam pattern and no edge illumination. Full beam would look normal.
If they are US assymetric they a totally illegal and highly dangerous as the dipped (cut off) beam will be directed into oncoming traffic.
That said if a prospective buyer had a set of UK xenon it would be possible to retrofit the UK projectors into them to benefit from the clear indicators. It's not diffficult/nor easy, but possible for a competent person.
caveat emptor