When crossing borders its not always a simple case of smiling and waving a passport.
A lot of places are corrupt including security and officials -- in this case theres two major problems:
A-- Consular assistance isnt there so he needs to find a solicitor asap.
B-- His actual embassy will be fighting tooth and nail to have him deported back to russia.
You cant make political statements thesedays outside of the UK and expect no blowback, the very least you can hope for in some places is jailtime hence the threats he has received hes done well to get out of russia.
You can get stopped at a border for almost anything and in that respect hes actually lucky to be in the UK now where he can have legal advice for free rather than serious problems.
Sorry for my ignorance but... Is this relevant?