Yes, strictly that is true, but in standard aviation parlance (in airport guides, etc) "Customs" means it is an international airport.
The first airport you fly to from the UK, and the last airport you depart from when returning to the UK, has to be a "Customs" airport.
In effect, therefore, the UK being in the EU makes no difference.
The UK works things differently from the rest of Europe, in that it has no "exit" Customs, and there are various concessions which are worked via the GAR form system. For example you can (IIRC) fly internationally to/from any farm strip, with appropriate (24hr??) notice.