Bear in mind that strictly the rules on specific construction are exempted for non business journeys of less than 50km, but you will still have to show that the animals are being transported in a safe and appropriate manner. Which means showing you can clean out (as in hose) the area they are transported in, get them in and out without risk of injury etc.
Personally if you are taking more than 1 animal in the trailer I would be making some gates, even if they arent compulsory, as it is very much easier to load and unload anything with less stress to the animal concerned and less chance of them legging it!