The reason the identities of the MPs remain confidential is the same reason the identity of any suspect in a criminal investigation remains anonymous. Until prosecutors have developed a case that they feel is ready to bring to trial, the investigation itself must remain confidential to protect the privacy of the people under investigation and to ensure that a fair trial can take place, should that come to pass.
The opposition parties - primarily the Conservative Party - is doing all they can to smear the Liberal Party with the taint of official corruption or foreign interference in much the same way that Trumpists are working to undermine the legitimacy of democracy by alleging that Democratic leaders are corrupt.