Chris, you are right. You get a 16amp breaker in the consumer unit, from which it will feed your 3kw element. You can fit both a 16amp timer (internal) probably under the consumer unit and an external 16amp weather proof socket. From that socket you just then run a length of 16amp extension cable to the replacement 16amp plug on your immersion cable in the van. That's it. Materials probably £50/60 and shouldn't be too much labour by the sound of it as you've not got far to go. It appears, from your description like a very straight forward job for any sparky.
Set the internal timer to whatever suites your needs and that’s it- plug in and away you go. Completely safe in the knowledge that you've done it safely and correctly. I've done Two this way now at different properties and it works flawlessly, simplicity itself!👍