Your cost will largely depend on your experience level. If you have over 3,500 hours, and over 500 hours of PIC in a multi-pilot airplane, you can self certify for the 14 exams. That means you are not required to register with an EASA ATO and formally take their courses. That will significantly reduce your costs. That being said, even if you meet those qualifications, I would still highly recommend investing in a good study guide and question bank. The U.K. has a well defined process and a lot of information about the process, but with Brexit, I would stay away from the U.K. unless that is where you wish to live and work. Many European airlines are telling their pilots to move their state of license issue from the U.K. as the relationship post Brexit is still not formally defined. The U.K. says they will validate EASA licenses, but EASA has stated they will no longer recognize U.K. licenses.
Anyway, as you stated, Norwegian is helping with the process. But I am not aware of any other airlines that are helping with the conversion. Despite what you may read on this forum, the long haul contract with Norwegian may not be a bad option. Their pay and benefits are actually in line with what many European airlines are paying and they have several bases around Europe (LGW, CDG, BCN, FCO, AMS and CPH).