As a result of recent terrorist plans to blow up nine passenger flights between Britain and the US, security advice has been upgraded across the board. Many flights from Heathrow have been cancelled, and getting in and out of the UK is extremely difficult at the moment.
News item here: http://www.abc.net.au/news/newsitems/200608/s1712219.htm
News item on arrest here: http://www.abc.net.au/news/newsitems/200608/s1712226.htm
DFAT advice here: http://www.smartraveller.gov.au/zw-cgi/view/Advice/United_Kingdom
QANTAS passenger advice here: http://www.qantas.com.au/info/ukTravelAdvice?lk=prau