The UK is not a centrally planned economy.
While Government can influence permission to develop and supporting infrastructure, most UK Airports are privately owned. It will be the Private Sector which leads the charge for a new hub airport, wherever it is located.
Heathrow has connections to Manchester, Edinburgh, Glasgow, Newcastle and, er, Leeds/Bradford. Rail or road satisfy most other demands. It’s a good thing that the UK can leverage Schipol as its alternative airport for regional connections; keeps the low margin, high pollution flights out of congested Heathrow.