It all goes down to politics. The Government thinks that there are more votes in being tough on border control even if it hits VFRs, businessmen, well of tourists and so on. It is said that UK tourism is loosing out because of high visa charges and not being part of the Schengen zone. A senior academic recently told me that the Gvnt would like universities to teach overseas students abroad because there is no danger of them overstaying their visa. How much revenue are we losing as a result of this? It might even make companies less likely to set up in the UK.
Bear in mind if there is a big outcry the Government will have to act. (It has introduced a penalty scheme for airport operators for weather delays even though the operators have rather less control over the weather!) If you are delayed write to your MP. Ditto if a friend from abroad is delayed.
As pinhammond points out, if you are really interested in security the important thing is to prevent undesirables getting on the plane in the first place. What they really needs is pre-clearance at the origin airport although I dare say that the logistics would be challenging. However details can be passed from the originating airport then all that you need to do is check the ID of the individual.
London is probably the international city par excellence but Britain is very insular. I suppose that it comes from being an island nation. Any passenger from outside the UK has to be rescreened because we don't trust other countries security checks. You would really think it wouldn;'t be necessary if the security of the originating country had been audited but no.
Its not just aviation though, complex checks are having to be made on Eurostar trains because domestic passengers travelling to Lille may continue to the UK. You really wonder why we can't sign up to Schengen but I dare say that the Government fears losing votes to UKIP.
Still as I said, write to your MP, the press whatever.
Rant over.