New Zealand is a good all round destination for people on gap years, especially if you're a first time traveller. Lots to see (fjords, amazing beaches, dolphins & whales, mountains, rainforest, volcanoes, penguins, Maori culture, kiwis...etc) and do (bungy jumping, kayaking, glacier walking, skydiving, skiing/snowboarding, hiking, white water rafting, general chilling out on amazing beaches). Plus the transportation options are fantastic - you can rent cars and campervans at 18, there is hop on hop off party buses that take you to the 'main' places, cheap public bus services, trains, hitch-hiking is accepted....Great party destinations if thats what you're afteer (Queenstown!) Did I mention the amazing beaches?
And if you want to make your friends jealous, you can be hanging out on the aforementioned amazing beaches in NZ's summer, which is Dec-Feb....during the UK's winter! Bonus.
You could also get a working holiday visa and prolong your stay with some casual work.