Pythagoras and the OSGB coordinate system

We've decided that, since sinϑ is very close to ϑ for very small values of ϑ, and we're only ever dealing with very small values, we'll simply use Pythagoras with the Latitude & Longitude. Which avoids any problems with operating internationally. That is, in areas within other countries where the areas are small enough for the accuracy lost to be immeasurable and/or irrelevant. We're not routeplanning or performing highly critical analasys, after all.