Shouldn't matter if it is UK or North American, Uk works on 11Khz banding and North American on 10Khz. Just means that sometimes maynot be able to lock onto the station for best reception.
Take it to a dealer as the antenna system is a diversity antenna system on the Lexus, e.g. it has two aerials, one on the top of the roof and the other intergrated into the back windshield. Could be either of them playing up or both, e.g. feed into the back of the audio system is damaged.