Connecting flights via airports like Stansted would be greatly complicated by having to go through security again.
If transiting then wouldn't need to go through security again as would stay airside.
Additionally would assume a Schengen area airport would do it first. It would not add anything to security because its assummed if there then have visa to be there anyway and that issue is an immigration one not an airline one provided they have vouched person could travel.
People joining from non Schengen would just join as they do now into many of the international airports when transiting.