Most cheap one-way flights from London to Glasgow use the other airports in the London.
Some of the routes are:
Glasgow International to London Heathrow
British Airways - www.ba.com
bmi - www.flybmi.com
Glasgow International to London Gatwick
Easyjet - www.easyjet.com
British Airways - www.easyjet.com
Glasgow International to London City
British Airways - www.ba.com
Glasgow International to London Stansted
Easyjet - www.easyjet.com
Glasgow Prestwick to London Stansted
Ryanair - www.ryanair.com
Glasgow International to London Luton
Easyjet - www.ryanair.com