Two useless secret agents are assigned the task of going undercover in Northern England in order to uncover the details of a disastrous terrorist plot being developed in a garden centre.

"Useless" seems harsh and an exaggeration. I would consider a different adjective. I don't find this logline enticing. Maybe on your next attempt, be more specific. What separates your spy/comedy (I'm assuming) from others? You don't need to specify the country this is set in. "Assigned the task" is redundant. Instead, put: assigned to go undercover, or simply, go undercover.

Also consider: if they're so useless, why are they being sent on the mission? Why not good agents? In fact, why are they in the agency in the first place? Think about recent spy comedies ... in Get Smart, Max is an expert in surveillance (reason for being in the agency), and goes on the mission because most of the other agents have been eliminated by CHAOS ... in Johnny English, Johnny is a suck-up paper-pusher (reason for being in the agency), and goes on the mission because he unwittingly allowed the bad guys to eliminate all the top spies at the top agent's funeral.

That's the setup for a plot, but not a complete plot summary for the purposes of the logline.

>>a disastrous terrorist plot

General. The threat needs to be specific.

>>>uncover the details

That's it? Just "uncover" They don't have to stop stop the plot?

What's the the specific objective goal?
What the stakes? What happens if they fail?