Don’t know how long this has been available, maybe it’s quite new, but now it seems you can get OS mapping on certain Garmin GPS’s such as the Nuvi 550, Oregon and Colorado. Its called Garmin GB Discoverer
http://www.garmin.com/garmin/cms/site/uk/lang/en/garmin-gb-discoverer
http://www.garmin.com/garmin/cms/site/uk/lang/en/garmin-gb-discoverer