Dorico uses the C4 convention for middle C (as does Sibelius, Finale, MusicXML, Scientific Pitch Notation and many others). Historically though, at some point sample library and DAW producers adopted the C3 convention. Most of the sample libraries I’ve looked at use the C3 convention, except for Spitfire and VSL. So ensure that the control in the Expression Maps dialog is set to C4 and don’t worry about the negative values - C-1 is valid in the C4 convention.
It’s pretty simple to set up the BBC SO expression map once you have the C4 convention and have note velocity for shorts and CC1 dynamic for longs.