When Danny finds dead Alex's body in London Spy, the police is quick to blame his death to a "BDSM session gone wrong". There is plenty of proof that Alex was a hardcore masochist, including a card from an exclusive escort agency.
In episode 4 Danny finds that indeed there was a history with an escort - but that this "professional" has been sent to seduce Alex.
The question is why the agency (or rather agencies, from MI6 to Mossad) decided to send a hooker to sleep with the guy who they were planning to kill anyway?
Was it to split Danny and Alex? But then we find that they have already broken up at that stage (but made up again later on).
Were THEY trying to control Alex via sex? Kind of doubtful, since we know that Alex was very good at figuring people out, so he would find that his new lover is insincere.
Was it just to add a piece to the murder cover-up? But then why to bother, they could just drop a dozen cards from different agencies without actually sending anyone to sleep with Alex.