Short answer: Nobody knows. And people who claim to know are either lying or don't know what they're talking about. But we can make some educated guesses. And if you ask again after the upcoming general election the guesses might be even more educated.
Currently the UK is set to leave the EU in january, but a Labour or Labour/Libdem government will probably ask for a new extension. And who knows for how long or if the EU will grant it (although my guess is that they probably will if a new government has a reasonable plan). A Tory or Tory/Brexit government might get parliament to approve the withdraval agreement or go for a no deal Brexit. If they approve the WA, UK will probably be in the transition period in April to May. But again, who knows. The Brexit saga has so far had more plot twists than any book I've read, and I presume there are more to come.
There are temporary air service agreements between EU and UK and USA and UK to ensure air service even in case of no deal and Eurostar claims to have made sure they can continue operate even in case of a no deal. I guess the ferry companies has done the same.
A government document on the impact of a No deal was published a while ago and can be found here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/831199/20190802_Latest_Yellowhammer_Planning_assumptions_CDL.pdf The document says worst case scenario, but on a leaked version it was claimed to be a most likely scenario.
If they leave in january the most problematic consequenses should (not will) have been sorted out by April. There will probably be protests, but I guess they will be centered around Whitehall and I'd be very surprised if they turn violent in Great Britain.