Technically it's not customs but immigration.
That's a good point that seems to be misunderstood often.
As the UK is part of the EU customs union, there is really no need to clear "customs" when flying to mainland EU countries. The customs union is about free movement of goods.
The UK are NOT part of the Schengen area - which is about free movement of people. Hence, upon entering the Schengen area, it is required for aircraft departing in the UK to clear immigration. In aviation speak, airports that offer customs clearance and/or immigration are often referred to as "customs airports" but that is quite misleading.
In contrast, if one were to fly in and out of Switzerland (a Schengen member but NOT a member of the customs union), one needs to clear customs indeed. No immigration required here.
The worst case is flying from UK to Switzerland or vice versa.
That requires you to clear both customs and immigration.