Summary: He received free money from EU. Now that he could not get enough from UK government, he is leaving.
For some companies, it's an indirect way to get funding, because it provides for cheaper highly skilled labour.
(Anecdote incoming) For example, many years ago at the end of my industry internship, the company offered to continue the project as a PhD, pursuant grant funding. When I declined, they offered a well paid full-time job to do the same.