Silverstrata,
For the government to pass a law you imply that LHR would be nationalised, it's owner, Ferrovial, would be compensated (at taxpayers' expense), and that taxpayers' money would fund the Thames airport and associated infrastructure.
How likely is that?
The Conservative government's policy in the 1980s was to privatise airports and those owned by the government were sold off. Conservative policy has not changed, Labour is as Thatcherite as the Conservatives, so they will not take this course of action. The Libdems and other parties will never get the opportunity to take this course of action, so it will never happen.
Apart from that, it cannot be afforded, and if it could, there are more important calls on taxpayers' money.
Your comment "I already avoid the place like the plague because of previous bad experiences" is quite revealing, it suggests that your arguments are subjective rather than objective.