We discuss matrix elements of the strangeness changing vector current using their relation, due to analyticity, with $\pi K$ scattering in the $P$-wave. We take into account experimental phase-shift measurements in the elastic channel as well as results, obtained by the LASS collaboration, on the details of inelastic scattering, which show the dominance of two quasi two-body channels at medium energies.The associated form factors are shown to be completely determined, up to one flavour symmetry breaking parameter, imposing boundary conditions at $t=0$ from chiral and flavour symmetries and at $t\to\infty$ from QCD. We apply the results to the $\tau\to K\pi \nu_\tau$ and $\tau\to K\pi\pi \nu_\tau$ amplitudes and compare the former to recent high statistics results from B factories.