"Take your ball and go home" - meaning of this odd phrase?

This refers to the childhood practice of playing a ball game with your friends, where the ball belongs to one of you. If the game doesn't go your way, you might get upset and leave, taking your ball with you, ending the game for everyone.

Essentially the quote is saying if Global withdraw from the deal, Orion Media will lose both Heart and Gold.

It's perfectly acceptable to use in informal writing.

When I was a schoolkid many years ago, some kids were just bad losers. We would play football in the park, and if their team was losing, they would take their ball and go home. It's the football equivalent of picking up the Monopoly board and throwing it across the room.