This dire warning comes as 330,000 troops carry out war games near North Korea in war drills jointly held by the US and South Korea.
The rival Koreas held preparatory talks Thursday for a summit later this month between North Korean leader Kim Jong Un and South Korean President Moon Jae-in amid a global diplomatic push to resolve tensions over the North's nuclear program.
China had largely sat on the sidelines as North Korea improved relations with South Korea, raising worries in Beijing that it was no longer a central player in the North Korean issue.
"The UK has only a limited ballistic missile defence (BMD) capability, however, if North Korea made a decision to launch a missile at the UK".
The report also suggested North Korea could be "dissuaded" from using nuclear weapons - adding the leader was "ruthless... but rational".
News reports said Kim is likely to be welcomed by South Korean military guards of honour, because two former South Korean presidents were met by North Korean honour guards when they visited Pyongyang for the previous summits in 2000 and 2007.
"It would result in the downfall - indeed the annihilation of the regime: the polar opposite of what Kim Jong-un is seeking to achieve".
The report, entitled Rash or Rational?: North Korea and The Threat It Poses, argues that the country has significantly stepped up its nuclear testing over the past 24 months.
Kim has since embarked on a diplomatic overture that has seen him pencil in summit meetings with the South and the United States and make his global debut with a visit to Beijing - his first overseas trip since taking power in 2011.
Defence Committee chairman, Dr Julian Lewis MP, said North Korea also posed a cyber threat and called for more money to counter it.
It said: "It is likely that North Korea has already successfully attacked the United Kingdom with the Wannacry ransonware, although we agree with the Government that the United Kingdom was probably not meant to be the principal target".
The WannaCry attack hit targets across the world, including the NHS, in what is thought to be one of the most damaging cyber-attacks yet in the UK.
"North Korea has shown an utter lack of concern about who gets hurt by such attacks".
Kim has since embarked on a diplomatic overture that has seen him pencil in summit meetings with the South and the U.S. and make his global debut with a visit to Beijing - his first overseas trip since taking power in 2011.
A time and place have not been set for the Trump-Kim meeting to discuss denuclearization.