Under UK Law, Japanese Knotweed is legally classed as a controlled plant under the Wildlife and Countryside Act 1981 section 14 (2) (WCA 1981).
Section 14 can be view here:
https://www.legislation.gov.uk/ukpga/1981/69/section/14
It is not illegal for you to have Japanese knotweed on your property, but it is against UK law to cause or allow the plant to spread in the wild or neighbouring land.