“What’s the relevance of Switzerland - they are to all intents and purposes a member of the single market so not like us at all surely?l
Wrong.
All goods are subject to customs agreements, or in many cases disagreements. The Swiss have never made formal agreements in many areas, mainly because they have frequent referendums and progress cannot be made, it’s their system. The EU don't want the same long running wrangle with the U.K.
Travel for foreigners is more or less open, but you might get stopped, we were and questioned about currency, but it was only cursory. Another day we crossed from Italy to Germany with no checks.
Free travel between the U.K. and Ireland is already agreed, there is no reason why that cannot continue, at present there are checks, you are checked at the airport and the ferry to mainland U.K., I was stopped at Holyhead and passport checked (maybe I look dodgy).
I’m pretty confident that commercial goods will have to be checked but not people, as now there will be many that travel and take advantage of shopping that is cheaper on which ever side of the border.