The article doesn't seem to touch on the motivation for the attack. (Perhaps it's taken to be obvious.) Why would the British government want to do this?
« The British spies appear to have targeted Belgacom due to its role as one of Europe’s most important telecommunications hubs. Through a subsidiary company called Belgacom International Carrier Services, it maintains data links across the continent and also processes phone calls and emails passing to and from the Middle East, North Africa, and South America. But tapping into a broad range of global communications is only one possible motive. GCHQ may also have sought access to Belgacom’s networks to snoop on NATO and key European institutions, such as the European Commission, the European Parliament, and the European Council. All of those organizations have large offices and thousands of employees in Belgium. And all were Belgacom customers at the time of the intrusion. »
And the second motivation is that they can. The pre-Snowden speculation was that all major European carriers are targets for NSA and friends, and the Snowden files basically reinforced that view. The question is not “why should they spy on their allies”, everyone has always done that; the question is the degree of success that any given player achieves and what they do with the info they gather. In this case, it looks like the operation was a great success, followed by huge failure (it was burnt to the ground).