It should be noted that it has not been 5 years since a Trident UGM-133 test firing. It has been 5 years since the U.K. performed a test firing as part of the recommissioning of one of our boats after a refit - but the USN also do test firings on a regular basis.
The UK does not own its own missiles, but shares a common pool with the USN held at King's Bay, Georgia and which are rotated on a regular basis. Hence the test firings are valid as a data source whichever boat or service fires them.
The last firing was by the USS Maryland on 31st August 2016 - and it was successful, the 161st successful firing since 1989.