This next plot shows the prevalence of land in cells labelled water in OSTIA. Here, blue colours indicate a small amount of land, and red a lot of land in the cell. The fact that the whole coastline tends to be fringed with colour confirms that the OSTIA mask is "fat" with respect to water. (That is, truly mixed cells tend to be labelled as sea, so there is a fringe of land-in-water-cell cases along coasts.) However, it is also clear that there are many "water" cells that are in LC CCI completely land (dark red in this picture). These appear preferentially on northern coasts. This is consistent globally, and not just around the UK. This suggests that there is an offset in the N-S direction, roughly half an OSTIA cell in size, in the OSTIA mask relative to the Landcover CCI data set.
Where an OSTIA cell is flagged as sea and is in fact filled with land, there will be (or should be) no satellite SST retrievals ever available for that cell -- the data will always be provided by the gap-filling procedures associated with creating the L4 SST CCI analysis.