I donțt know about Brexit, but I think Musk hinted years ago that he might do a Gigafactory in Germany or France. The reason for this is because that's where all the car workers are, and Musk will want to be able to attract that talent.

Germany was about the best option, except that it was also a bit higher risk, as Merkel has been catering to the old carkamers quite a bit, often with moves against Tesla. But now that VW and other German carmakers are shifting a little towards EVs, I guess that risk isn't as great anymore.