Add some electric motors to the tricycle to drive the back wheels and add a tow bar and off you go. Could take a bit longer to get to L-O-S and you may need to stop to re-charge but no need for insurance or petrol.
I had a look at this a while ago when I was thinking of building a trike, as a better way to get to the sailing club than a bike or car. Provided the motor is less than 200W, cuts power when you stop pedalling and has a controlled top speed of 15mph then you do not need registration.
The thing that I thought would be the most auquard to build (apart from the diff) would have been the control system, all the eisily availble options seemed to be either overly complex or likely to be low reliability, or draggy when not using the electrical assist.