This looks like a measure in absolute numbers so Fiestas etc appear high on the list simply as they are one of the most common / popular cars in the UK by virtue of numbers (which also drives a lucrative trade in being stolen for parts).
Normalised data is a much better comparison eg thefts / number of vehicles so a direct comparison can be made. Lexus make up only a small proportion of cars in the UK, so in absolute numbers the number of cars stolen may look small, but the actual incidence of theft (or likelihood of your car being stolen) is much higher.
Whilst writing the above just received an email from LV, extract as below.
"Over the past year, there’s been a rise in thefts of the LEXUS RX 450H F SPORT AUTO. As a result, we’ve seen a large increase in the number and cost of claims we’ve received for these types of vehicles and have made the difficult decision to stop insuring them."