According to the RHS it is now high in the top ten of garden pests. It came to Britain from Southern Europe in the mid 1990s and has since become widespread in London and East Surrey.
Outside of the London area, the insect has become established in Norwich and a few adult specimens have been found in Leicestershire and South Yorkshire. I can't find any records of the beetle in Kent apart from a sighting in Medway but that may be down to under-recording as is often the case with coleoptera etc.
The rosemary beetle is fond of rosemary and lavender, but is also partial to thyme and sage. The advice is to squash any found but how could anyone destroy such a beautiful thing? Although my sighting is late in the year, these beetles will be out and about on warmer winter days.