Using OSM and search engines to find postcodes

OpenStreetMap has tons of road names, those road names exist within some town, the town exists within a county. If we search the web for "road name, town name, county, " and then read and validate the next few characters from the search result we can find every postcode that has a business on it, without infringing on Royal Mail's database rights.

By using the mean location of the waypoints along the road, we have an approximate GPS location.

Does this sound feasible, and legal?