Its basically the UK Government to blame for this.

The Government made the decision to sell Heathrow Airport as a one business; not by separate terminals like the rail network. This was bought be Ferrovial as was Gatwick. These was placed in a company called BAA (British Airport Authorities). It was renamed when they had to sell Gatwick.

Ferrovial own several airports around Europe. Their business model is to have two sources of income, landing fees to the airlines and a share of all profits from the retail outlets in the Terminal. Ferrovial organised the financing for constructing Terminal 5 (and now Terminal 2) BAA will not rent the Terminal 5 structure to BA as it would be too expensive to give up their share of all profits from the retail outlets in the Terminals. Also, Terminal 5 was built which so much technology incorporated, that an tenant would be nervous to take it all on themselves.

The Government also chose to outsource their responsibility for airport security so they cannot be blamed for any failings. The US government chose to take responsibility for the TSA. This is the cause of issues with Fast Track and airport security since BAA or the passenger or the government has no direct contract with the management of the security management team. All the can insist on is a Service Level Agreement where there are in-built allowances for deficiency from time to time. Even if the Service Level Agreement is not met, no compensation or recourse is due to the passenger or BA.

In fairness, it is Ferrovial's own interest to get passenger through security asap to maximise passenger's time airside. However the cynic in me believes that "if Ferrovial make the process to efficient, passenger will delay their arrival times at the airport. So if they make their security screening erratic, passengers will allow extra time and end up having more time to browse the shops".