We present a novel, physically-motivated sub-grid model for HII region feedback within the moving mesh code Arepo, accounting for both the radiation pressure-driven and thermal expansion of the ionised gas surrounding young stellar clusters. We apply this framework to isolated disc galaxy simulations with mass resolutions between $10^3~{rm M}_odot$ and $10^5~{rm M}_odot$ per gas cell. Each simulation accounts for the self-gravity of the gas, the momentum and thermal energy from supernovae, the injection of mass by stellar winds, and the non-equilibrium chemistry of hydrogen, carbon and oxygen. We reduce the resolution-dependence of our model by grouping those HII regions with overlapping ionisation front radii. The Str{o}mgren radii of the grouped HII regions are at best marginally-resolved, so that the injection of purely-thermal energy within these radii has no effect on the interstellar medium. By contrast, the injection of momentum increases the fraction of cold and molecular gas by more than 50 per cent at mass resolutions of $10^3~{rm M}_odot$, and decreases its turbulent velocity dispersion by $sim 10~{rm kms}^{-1}$. The mass-loading of galactic outflows is decreased by an order of magnitude. The characteristic lifetime of the least-massive molecular clouds ($M/{rm M}_odot < 5.6 times 10^4$) is reduced from $sim 18$ Myr to $<10$ Myr, indicating that HII region feedback is effective in destroying these clouds. Conversely, the lifetimes of intermediate-mass clouds ($5.6 times 10^4 < M/{rm M}_odot < 5 times 10^5$) are elongated by $sim 7$ Myr, likely due to a reduction in supernova clustering. The derived cloud lifetimes span the range from $10$-$40$ Myr, in agreement with observations. All results are independent of whether the momentum is injected from a spherical or a blister-type HII region.
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