Airlines "forced" to use STN
Forget about airlines being "forced" to use STN instead of LHR.
First, in a private sector and deregulated industry, the government has no right and no business to determine who goes where, and years of court cases and litigation would render any such attempts futile. To enforce this policy, a certain amount of public ownership would be necessary, and there is no political appetite for this.
Second, airlines at LHR are there because of the available connectivity and premium business. This is how they make their money (pax down the back are the icing on the cake). These are not, and never will be, available at STN. Carriers denied access to LHR will go to where these are available: AMS, CDG and FRA.
It is as simple as that, and it is not just STN, It applies equally to LGW, airports in other parts of the UK, and to any proposed estuary airport.