Arguably Monzo’s closest rival is Starling, as they share Monzo’s business model too & are based in the UK but assuming that N26 move into the UK or Monzo expands into Europe, as they plan to, they will definitely be a competitor.
Having seen some of the issues that I’ve seen reported on Twitter, I’m not sure they’re “the benchmark” but he definitely has said that they’re a year ahead of Monzo.
I don’t know the timescale but I expect you’re right that they’ll enter the UK market at some point.