They had a serious investigation going, but that was during Boris Johnson’s time as prime minister (BoJo = the Trump-impersonator with the silly hair), so it was pretty handicapped and when the investigation stopped, well:
Johnson’s government refused to release the report to the public before the general election in December 2019.
By June 2020, the report had still not been released, and the Intelligence and Security Committee had not been convened, the longest gap since the committee’s creation in 1994.