OK maybe an example would help.
If I had a lv commercial installation using all schneider gear with a key switch controlling an acti 9 relay controlling lights. All wiring installed to bs7671. Do I need to worry about ce certification?
If this is a stand alone control panel, then yes, and BS 7671 would not be the relevant standard. If it is for lighting it would be the EN 61439 series, and you must CE mark in accordance with the Low Voltage Directive.
If it is to control machinery then the design and build standard would be EN 60204-1 (for general machinery, there are a couple of specific machine types that are outside this).
Using all CE marked components does not make the assembly CE marked.
I can take you through every reason if you like, but we'll be here a very long time, and I'm off to China Saturday for a few weeks, to advise a Chinese machinery builder on CE marking of their machinery and electrical panels.
I do much more of this now than I do electrical works, as my background is in machinery, and I used to do this before redundancy and self-employment.
What Rob has said is correct.
If you are designing and building electrical panels, there is a lot more involved, as the panel becomes a product in its own right, and that must be CE marked.
If it is controlling machinery, then the assembly of panel and machine must comply with the Machinery Directive.
This is the same concept as when you mix and match MCB's in a distribution board, like it or not, when you do that, you become the manufacturer of the assembly in the eyes of the law.
Oh, and as of the end of March no matter what happens, the only thing that changes will be CE being becoming UKCA marking for the UK market.