Graham, I halter-trained my first Jersey calf using the pressure-and-release method no bother.
My older Jersey - her mum, bought as an in-calf heifer - has never taken to a halter, but accepts a neck strap.
The Tinkers' Bubble Jerseys were halter-trained and were led from field to field this way, just as you would a pony.
All my Jerseys more-or-less follow me / come when called, no halters or straps required, because I usually mean good things.
However, I would not rely on this on the road, no. When we need to move our cattle on the road we have one of us in front stopping the traffic and the other behind, shoo-ing the beasts along and stopping traffic driving up their backsides. With quiet cattle on reasonably quiet country roads, this works fine with suckler cattle - it should be no bother with Jerseys, I'd have thought? Unless you're talking miles, of course, or major roads.
As to getting them into the trailer - all my Jerseys would load no bother at all if they were used to getting cake in the trailer