If I'm using this head model as the default anatomy rather than individual subject anatomy do i need to compute the BEM surfaces or have they already been computed?
You should be able to use the BEM surfaces available in the templates: outer_skin, outer_skull, inner_skull.
In addition, can I compute FEM surfaces from this head model?
I don't know how the algorithms for computing FEM meshes work on these templates, I haven't tried it.
You can try the various solutions available: https://neuroimage.usc.edu/brainstorm/Tutorials/FemMesh