Young, massive stars dominate the rest-frame ultraviolet spectra of star-forming galaxies. At high redshifts (z>2), these rest-UV features are shifted into the observed-frame optical and a combination of gravitational lensing, deep spectroscopy and spectral stacking analysis allows the stellar population characteristics of these sources to be investigated. We use our stellar population synthesis code BPASS to fit two strong rest-UV spectral features in published Lyman-break galaxy spectra, taking into account the effects of binary evolution on the stellar spectrum. In particular, we consider the effects of quasi-homogeneous evolution (arising from the rotational mixing of rapidly-rotating stars), metallicity and the relative abundance of carbon and oxygen on the observed strengths of HeII (1640 Angstroms) and CIV (1548,1551 Angstroms) spectral lines. We find that Lyman-break galaxy spectra at z=2-3 are best fit with moderately sub-solar metallicities, and with a depleted carbon-to-oxygen ratio. We also find that the spectra of the lowest metallicity sources are best fit with model spectra in which the HeII emission line is boosted by the inclusion of the effect of massive stars being spun-up during binary mass-transfer so these rapidly-rotating stars experiencing quasi-homogeneous evolution.