Prior to November 1989, there was a single driver licensing category which covered driving a car and driving a car with a trailer. In that month, new regulations split this into two categories, one for a car and one for a car with trailer. The requirement to do a test in order to have the trailer category added to a licence has existed since then.
People who held a full licence before the change were entitled, when renewing their licences after November 1989, to retain the right in relation to trailers. It appears that some people applied for the car category only, and therefore unintentionally gave up their trailer entitlement.
In 2013, in response to representations from farming organisations, my predecessor decided, on a once-off basis, to allow people who had held a full licence before November 1989 and had mistakenly surrendered their trailer entitlement to reclaim it. The time period for this once-off allowance has long since passed and I have no plans to repeat it.