Not by Law, and we all know that labelling of provenance has regularly been by-passed in the most of inventive/deceptive of ways.
Maggiemaybe- quite simple. Watching the demise of the UK and how isolated Brexit has made it, in 100s of ways, from supply chains, to provision of essential materials and chemicals, fresh produce, medicines, etc- the closure of so many large businesses and industries, the shortages of staff in so many fields, the loss of cooperation for education and research, and innovation projects, the financial sector, and on, and on.
They have realised that United they succeed and can provide each other with all essential goods and services, as the countries are so varied and produce stuff that others can't, and vice-versa. But also it has strengthened the concept of EU cooperation and strength. Even for the vaccine- working together took a little longer, but they were then able to procure huge numbers of vaccines at a much much lower price than UK going it alone, for instance.