http://www.mcga.gov.uk/c4mca/brown-withpage8.pdf
Its a long document, but worth reading through and understanding the basics.
If you are building it and intend to actually get it coded, you'll need to follow it to the letter, plus take advice from your surveyor.
If you just want the boat to be near to coding as reasonably possible (as I did) read it, makes notes on the pertinant areas and do your best.
As you mention the basics are things like rail heights, upstands on deck hatches, saftey rails and wires. Things like nav lights need adhering to even outside of the COP.
For our sized boats it is all pretty simple.