In the UK-minimal foam. A tiny head. On mainland Europe-at least a quarter.
Don't you guys drink warm flat beer anyways?
For a country that invented the IPA
[1], I couldn't find one over there that wasn't just a sorry shadow of a beer, likely poisoned, and a complete and utter waste of time and calories. Horrible stuff. Finally found a Dogfish Head 60 minute after about a week, and was in heaven.
If you like IPAs and visit the UK, just enjoy their various ciders and adequate-enough ales; save the IPA as a treat for your return to the more civilized world.
1 - hops are a natural preservative, so the traders would over-hop their ales in order to keep them fresh for the journey to India, and the ales came to rely more on the hops and the light roasting, hence pale ales for India, aka India pale ales, aka IPA.