Honestly all this panicking! We are not going to be stopped travelling around the globe!
The UK has decided that it wants to lose the benefits of being able to travel freely around Europe - it is not a decision made by the EU. Yet again, Brexiters seem unable to grasp the concept that voting to end the ability to freely and easily travel means that travel will no longer be free and easy.
We are not going to be made prisoners on our little island.
One of the conspiracy theories is that Brits are indeed going to be made prisoners on a fenced-in island. The Berlin Wall / Iron Curtain was not built to keep the foreigners out of the Eastern Bloc - it was intended to keep the population in. Even if that sounds a bit far-fetched, there are those of us who are old enough to remember that there were limits on the amount of currency that one could take out of the country, which in effect limited most people to one short trip abroad each year. If the pound plummets and people look to move their assets out of the UK (ask Redwood or Rees-Mogg for more on this) it is far from unlikely that the government would impose similar restrictions in 2019.
The worst that will happen is we will find our selves in a slower immigration queue and we will need to either apply for a visa online or fill in a paper document on the plane to identify who we are and where will be staying whilst holidaying.
The worst that can happen is that British citizens cannot leave the UK until some sort of agreement or system is put in place. (Ignoring for now the issues with aviation and ferries etc).
'The slower immigration queue' - 240 people arriving on a plane, each taking 5 minutes at the Immigration Desk, would represent 20 person/hours of work - i.e. with 20 officers on duty, it would take an hour to clear the queue. Many places I fly into have a maximum of 5 immigration desks, often fewer. It might take 4-5 hours to clear just one plane-load.
There is no online service for applying for a Schengen Visa - it is still being developed with a planned 2020 introduction date, and it was never scoped with the issue of 58 million UK visitors in mind. Likewise, what system will be in place in the EU to print, distribute, collate and check 58 million pieces of paper?