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Where on earth did you get that from?
There's no requirement to notify customs of a departure from the UK to an EU state, wherever you depart from. For the return, you do need to notify customs and immigration. This used to be done through the CCUs, though the NCU took over this role. I've not seen a statement of the notice required -- most CCUs wanted 4 hours, and I'd be surprised if it were 12. It's about time the AIP got revised to reflect this.
Special Branch do not need to be informed of flights to France, though they have the right of inspection of any such flight (more recently, any flight at all) on arrival or departure if they choose to be there. The notification requirement only applies to flights to Ireland, IOM or the Channel Islands from non-designated airports.