I'm just on an ordinary EE SIM-only plan. Not one of the current ones, but they change constantly anyway (for which reason I occasionally check what's on offer when I'm out of contract, as I am now, since sooner or later I come across a promotion which offers more than I currently have for less money).
The cap when in Germany or Denmark is the same as it would be in the UK. But as I've noted previously, I wouldn't try to live there permanently on it because I've read the EU-wide rules of the EU roaming agreement and have no reason to believe they'd make an exception for me. But as I'm UK-based but (under normal circumstances) a regular visitor to a number of EU countries, that isn't one of my requirements, the regular EU+EEA roaming deal for visitors meets my needs fine. Most of my travel is to laboratories or universities, so I generally have WiFi available anyway (I'm always amused that Geneva Airport is an Eduroam site these days).