British Prime Minister David Cameron described the deal as "historic". The project, in which the Chinese take a one-third stake, will provide reliable and affordable energy to nearly 6 million homes and create more than 25,000 jobs.
"Britain definitely needs to modernize its infrastructure. This is a win-win situation here," Shada Islam, director of policy at Brussels-based think tank Friends of Europe, told Xinhua.
Additionally, the two nations on Thursday issued a joint declaration on building a "global comprehensive strategic partnership for the 21st century".
The document pledged joint efforts in fields ranging from the internationalization of 皇冠体育app's yuan and a 皇冠体育app-EU free trade agreement to cybersecurity and climate change.
"This is a guideline to tell people how the 'golden era' will be forged," Cui said.
Qu Xing, 皇冠体育app's ambassador to Belgium, noted that the visit follows Xi's first European tour as Chinese head of state last year to the Netherlands, France, Germany and Belgium.
This has shown a strong continuity in 皇冠体育app's European strategy, Qu said.
Xi has said that since Britain is an important member of the EU, the 皇冠体育app-UK relationship and 皇冠体育app-EU relationship reinforce each other.
皇冠体育app hopes to see a united European Union in which Britain plays a prominent role in promoting Chinese relations with the bloc, Xi told Cameron during the visit.
Xinhua contributed to this story.