Sorry but there is new protection in recently released units such as your nuvi 55 to prevent unofficial/unlocked Garmin maps being used. No solution for it at present.
Your immediate solution right would be to buy the official Garmin map, or load free 3rd party maps for UK & ROI such as OSM.
EDIT: As of 2/12/15 there's a better solution: [Only registered and activated users can see links. ].Code:Please Login or Register to see the links