OS only makes raster maps, there are no good GB Topo (vector) maps made by Garmin. The last and only one was Great Britain Topo v2 but that map was very bad and is from 2007.
There are good alternatives however, for your motor bike I would suggest to use City Navigator and good Topo maps are based on the Openstreetmap project.
The best Topo map available in my opinion is the German Freizeitkarte, it's free and gets updated every 2 months.
You can download the map of Great Britain or the entire region of GB and Ireland.Code:Please Login or Register to see the links