It is a big mix. Some Brexits have deluded themselves into thinking that that Galileo is an ESA project and as ESA is more than the just EU, the UK could stay with ESA and remain on the inside track of the Galileo project.
ESA plays an important part of Galileo and provides some of the funding. Galileo was supposed to be funded by a public/private partnership. As US GPS is free, the private side saw no way to get a return on their investment so kept their cheque books shut. The EU decided they needed an accurate GPS that the Americans could not switch to approximate mode (or off). They found the bulk of the money. If the UK is not in the EU, then the same logic that funded Galileo implies that the EU needs a GPS that the UK cannot bugger about with.
When the UK voted Brexit, every other state in the EU saw an opportunity to divide the UK's share of EU pork among themselves. As the UK has negotiated away its voting rights, we can expect future negotiations to go like this.