CE certification: battery-powered LED lights
I've self-certified products with AC, on-line, switch mode PSUs by precisely following manufacturer's guidelines on on PCB layout and component recommendations. If you self-certify you have to produce documentation (a technical report seems the best way) that justifies why you have not done a full, third-party tested EMC test and I've had no qualms in doing this.

On other jobs, I've done informal 3rd party lab testing on certain parts of the design to ensure that emissions were OK and documented results in a technical report. I've never bothered with doing susceptibility testing because, in my experience, using a relatively inexpensive static-discharge gun finds all the susceptibility weaknesses in a design that may brain may have missed.

This approach does require an almost certified design for a switcher but this can usually be gotten from a lot of chip suppliers.