Sorry for this question, but its based on logic......
Schengen is an immigration control agreement (not customs). Last time I looked UK was part of the EU customs union.
Logically, therefore, from a customs perspective, UK to mainland Europe should be no different from Slovakia to Czech or Germany to France. No requirement for customs.
Being outside of Schengen does require an immigration control on the transit though !
If this is being operated differently by the customs agencies, then it needs to be challenged at the EU level.
As I only normally cross mainland boundaries, please advise if you feel you are being required to undergo custome checks on UK-mainland Europe flights ?