Britons may have to apply to visit Europe after Brexit

"In theory UK citizens, as third-country nationals, would certainly be subject to the obligations" of such a scheme, said Camino Mortera-Martinez, a research fellow specialising in justice and home affairs at the Centre for European Reform. "This will have to be part of the Brexit talks. It will all have to be negotiated."
...Mortera-Martinez said Etias “may be some time coming” and stressed that that in practice Britain’s future deal with the EU would almost certainly include some form of provision for free travel – as opposed to the right to live and work – on the continent. But the free travel question will be one of many bargaining chips.

"Britain is a neighbouring country, it is a safe country, this would be very bad for business and there would always be a possibility of retaliation," Mortera-Martinez said. "But this question will have to be dealt with. The two are not remotely comparable of course, but after Brexit Britain will be a third country like Turkey."