There is however land reserved for a high-speed London-Birmingham with (IIRC) either a branch to LHR or via LHR. You can see the land alongside the line from Birmingham Moor Street to Marylebone - certainly from Moor St as far as Leamington Spa. It is part of what is known as HS2 - HS1 is already built and is the Folkestone-London channel tunnel high-speed link.
Well, I wouldn't call it reserved land
I'd call much of it the old four-track alignment of the Great Western Main Line from London to Birmingham. Whilst there are no housing developments on the alignment, there are a few places where the land has been put to another railway related use, but those are easily removed. i.e. a set of mobile towers and a park and ride car park at Acocks Green.
Also all because the land is reserved, doesn't mean anything is going to alter. Some land by my parents home was reserved for about 25 years for the Birmingham Western Orbital. That never got built.
One of the interest effects of the Greengauge scheme is that BHX becomes LHR's 3rd (or 4th) runway.