1. Greece owes €9.4 billion to the UK. If they failed to pay what they owe our own economy would suffer.
2. A Greek default could lead to a massive fall in the value of the Euro. That would hit business confidence and share prices worldwide, probably plunging many major economies into depression. (That's why the US authorities are particularly keen to see a deal done. While Greece only owes the USA €6.4 billion, the collapse of the Euro could have major knock-on effects to the US economy).