Representation for the UK government:

if a "cross-border" employer becomes insolvent the critical point to consider in determining which of 2 or more EU countries must pay up under the state guarantees of unpaid wages required by EU law is in which Member State(s) did the employer have a stable economic presence. In the case of a transport undertaking established in a Member State, the mere fact that a worker employed by it in that State delivers goods between that State and another Member State does not demonstrate that the undertaking has a stable economic presence in the other Member State.

For relevant general notes see European Law/DIRECTIVES etc/insolvent employer