Britain has been looking to build global trade ties following its departure from the EU in 2020 and has looked to pivot toward geographically distant but fast-growing economies.
It is deepening its ties in the Indo-Pacific as its foreign policy framework casts China as an "epoch defining challenge".
Other members of CPTPP are Brunei, Canada, Chile, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam. Britain is the first new member to join the group.
Membership will supplement existing bilateral Free Trade Agreements (FTAs) Britain has with most of the member countries, and gives businesses extra options over the terms they can trade under.
The overall impact of the trade deal is set to be modest. Britain said the deal, which will cut tariffs on cars, spirits and dairy products, would boost the economy by 1.8 billion pounds ($2.2 billion) each year in the long-run - a figure that could rise as more countries join the pact.
"The Pacific Rim is forecast to grow twice as fast as Europe, so UK businesses should be thinking about their regional presence," said Sally Jones, trade policy and strategy partner at EY.
