SLF - I suspect the answer is a pragmatic one that goes a bit like this. As your carry on will have been security screened at T1 or the FCC, it won't have anything particularly dangerous in it and the screening will itself be a deterrent to carriage of certain types of contraband. And Customs just aren't interested in the small leakage that might still get through this route, considering that such transfer pax are a relatively small proportion of total London arrivals and what they can smuggle in cabin baggage alone is very limited. Customs are largely playing a numbers game and they know what they're really interested in.
Agree with Globaliser on this. A couple of transfer bags off a flight to Customs is a low priority compared to a Las Palmas flight with 330 passengers, 20 of whom may have huge excess imports. It's a staffing, risk assessed numbers game.
There is a distinction to Security screening of all hold baggage and rescanning of transfer baggage which is a legal requirement stipulated by the Department for Transport and Customs screening.
They are two completely separate issues. Security doesn’t care if you have 600,000 cigarettes in your bag. They will probably give Custom's the heads up however.
In reality to the passenger he or she really doesn't know if their bag is being checked by Custom's or not so has a similar effect to mandatory screening of all bags for customs issues.
After all it's protecting our taxes not a security risk.
The scanning of bags for security reasons is so strict because sadly this could have prevented Lockerbie. The bag from Lockerbie was a FRA transfer bag loaded at LHR without rescanning.
All dispatch, loadering etc staff have it drummed into them that they don't accept hold to hold transfer baggage. If there is not enough time to screen this baggage before the onward flight departs. The bags stay behind and the flight operates without them. That’s why there are so many issues with late short connection bags.