British expats: no longer EU citizens free to work, set up businesses, study, retire in EU countries nor benefit from subsidised health services across the EU. The Euro Health Insurance card 'EHIC' would cease to be available, so emergency health cover from Britain would cease. Private health cover would be mandatory but not available to those with pre-existing conditions!
UK citizens: risk Britain's current buoyant economy plus future investment possibilities, widespread job losses, smaller ranking in the world, lose 28 EU friends, jeopardise the current 70 years of no war in Europe. Imagine a sudden terrorist attack with no friends. If it ain't broke, don't fix it!