Stipulated, they screwed up, but why?
First, it's not coincidental in happened to a Hawker crew in Ireland. Charter operator and crew from US, who rarely fly outside of North America. Do we know their training? No. Do we know what they flew in Europe prior to this flight and how they performed before this departure? No. They could have never done a crossing until they picked up the plane at EIKY. OTOH, they could have flown around Europe all week with nary a problem until thrown off by FL Two Hundred.
True, they shouldn't have turned a wheel confused about the airways clearance, but flying a crossing from a foreign country, if you rarely do it, is daunting.