We’ve just finished another site which embeds OS Openspace maps into WordPress pages: Dartefacts. This uses a custom post type to record the grid reference of an object, which is then displayed on a home page map showing all the items, custom taxonomy pages and search results. This works really well with several hundred items (so far) displaying on the home page, and is fully responsive as well.
We’re really pleased with this site (and so is the client!). This builds on our earlier site Rombalds Riding which displays gpx routes as well as points.