Using inclusive deep-inelastic scattering cross sections from the H1 experiment at HERA, electroweak parameters of the Standard Model are determined accounting for their correlation with parton distribution functions. The cross sections were measured using longitudinally polarised lepton beams, which enhance the sensitivity to the vector couplings of the light quarks. The quark couplings and the electroweak mixing angle are probed through the $\gamma/Z$ interference. This gives access to electroweak parameters in $t$-channel exchange at virtuality up to scales exceeding 10 000 GeV$^2$.