If you look at MAN on Google earth is quite apparent there is ample space around the airfield to grow significantly beyond its current foot print.
There is the small matter of the green belt and planning control to overcome. As such whilst there is space to expand in the current land use plan, it will require displacement of facilities such as the cargo and maintenance areas.
Re LPL. It may suit LPL's economy if you look from a purely local point of view, but if you expand that view from local to a regional, then MAN growth would be far better for the entire NW region and economy than the benefits to the region that an enlarged LHR would create.
So by extension, taking the national view expanding Heathrow is better for the UK economy as a whole, but that is not an argument subscribed to by most on this thread. You can't have it both ways...