According to this map released by the Foreign Office, there are 18 countries which you should probably avoid holidaying in.
Unsurprisingly, a good few of them are in the Middle East. Seven to be precise: Afghanistan, Iraq, Libya, Syria, the Palestinian territories, Tunisia and Yemen. So that lad's tour you were planning around the region may have to wait...
The rest of the countries on the DO NOT TRAVEL list are:
Central African Republic
Democratic Republic of the Congo
The FCO also listed 45 other countries which they consider partially dangerous, including the northern Sinai and the Western Desert in Egypt. For now, Sharm El-Sheikh in the southern Sinai has been given an "all but essential travel" rating, following last weeks disaster.
The other countries to check against the FCO website before you book your flights are:
Japan (around Fukushima)
Republic of Congo
And just FYI, if you find yourself in Syria anytime soon the UK government won't be able to get you out. They're currently advising that you leave 'by any practical means' possible.