That depends on what you mean by the word 'business', doesn't it? In Amazon UK's case, for example, the 'business' as in the legal offices of the company are in the RoI. But the 'business' as in the act of buying, selling and and delivering goods, happens very much within the UK.
The could in theory move the former to anywhere within the EU (I think) without changing very much legally. However if they want to move all of the latter out of the UK, they're going to see something of a drop in their financial performance.