According to Ms Annesley's Twitter profile, the aspiring writer was travelling solo as part of a "dream trip" to Thailand.
Ms Annesley was believed to be in a relationship with Olly Neville, a former leader of Ukip's youth wing who was sacked in 2013 after saying he backed gay marriage.
A Foreign and Commonwealth Office spokesperson said: "We can confirm the death of a British National in Thailand on 21 January 2014.
"We are providing consular assistance to the family at this difficult time."
Online tributes flooded in to the young girl, who was a vocal political activist during her time studying at Leeds University.
Will McHoebag tweeted: "Still lost for words. I am absolutely devastated. We will miss you very much. She was fantastic."
Camden Conservative Deputy Chairman Oliver Cooper wrote: "Devastated by the sudden and untimely death of friend and freedom-fighter Christina Annesley. I didn't know anyone that didn't adore her. RIP.