Lax_lhr
I assume the £450m you refer to is the amount recently raised from the MAG bond issue. If so, this was simply a refinancing of existing debt (see quote below), and is therefore not available for new investment at MAN or STN. Also there is no cash left over from the share sales as this was used to fund the equity portion of the Stansted investment. But that's not to say that MAG can't raise additional finance, for example by issuing more bonds.
MAG lands in the bond market
Manchester Airport Group inaugurated its new £5bn debt programme with a 20-year bond issue, refinancing half of the term loan it used during its acquisition of London Stansted airport last year. This deal establishes it in the capital markets, and gives it the flexibility of funding options other UK corporates enjoy.
IFR 2019 8 February to 14 February 2014