On the run from a determined street enforcer, a young Londoner will do anything and everything to avoid capture but the enforcer has pledged to kill himself in the event of failure.

Needs to be more concise.

Thanks for your feedback. Switched Order. Chiseled Vocabulary. Added Verbose. :)
Cheers ;)

If the Antagonist vows to kill himself there is no Movie.
As above said there has to be some connection, change of heart that makes it an option to change direction.
Why does he have to out run this Enforcer.....What about the boss?

EdgeWriter is right although maybe add that the person on the run finds out that the street enforcer is related to him/her and feels sympathetic??, let's himself get captures to avoid death to the enforcer.
Not sure but hope this helps

Don't know if this has been mentioned because I haven't read all the comments, but two issues strike me about your logline:

1) how and when does the assassin decide he's failed
2) the Young Londoner simply has to hide out and wait for the assassin to kill himself

>>aren't two Hollywood studio gods predicting doom?

Their doom, perhaps, as a new generation of junior gods storm Mt. Olympus. Who knows? The only constant in Showbiz is change.

Anyway, your premise keeps growing on me [like the man-eating plant in "Little Shop of Horrors"] :)

Best wishes