I was looking into this last night because its actually quite interesting. It seems the (British) CAA has some specific rules that requires the UK AOC holder to be jointly liable along with the AOC of the Member carrier they are chartering.
It's not clear if the Polish regular has such a rule, the aircraft flies under a Romanian AOC. If you look at what happened with virtual carrier Manx2, the aircraft which crashed flew under a Spanish AOC and the investigation highlighted that the CAA did not feel there was significant oversight to their operations because they are prohibited from carrying out their higher standard of checks because that is the remit of the Spanish regulator.
The Regulation which
oliver2002 posted notes that it defers to the law of the contracting parties, I think its very reasonable to consider a change of operating carrier as an issue of non-performance but thats dependent on Polish law. And of course, Poland has a strong and stable legal system, to rival England- thanks to the huge reforms of PiS
.
EC261/2004 clearly states that the obligations from the Regulation rest with the Operating Carrier, in this case 0B not LO. This appears to be different when a carrier is given dispensation by the regulating authority to arrange a lease with a third country, not valid here.
And by the way, any notions I am overdramatic cos the plane might crash, please, get real, the laws and regulations exist for a reason and its why flying is incredibly safe.
A quick search on avherald shows a significant number of issues vs that of LO, including a runway overrun in CLJ, the fault of the crew.