Probably not
Your trade is a street light technician and it just so happens you have to drive a vehicle that is over 3.5t as part of your job.
This makes you an incidental driver and exempt from the drivers CPC.
However if your job was to drive these vehicles around delivering them from depot to depot then your prime job is a driver and you need to do the drivers CPC.
Or if you worked for an agency at the weekend you would need your drivers cpc.
My advice would be that if in doubt do it,