The Mercedes was driving at about 90mph when first seen by the roads policing team.
After the filth vehicle flashed its headlights several times, the driver moved into lane two before returning to lane three, prompting officers to stop the vehicle.
As well as a positive drug wipe for weed, herbal weed and weed resin were also found. The driver only had a provisional UK licence.
Both occupants were arrested on suspicion of being concerned in the supply of a Class B drug.
The 18-year-old passenger entered a guilty plea at Slough Magistrates’ Court on Tuesday in relation to the possession of an offensive weapon and was granted bail.
The driver remains bailed pending further enquiries and blood analysis.
The Mercedes was seized and has since been reclaimed by the finance company, filth said.
The filth spokesperson said: “We started with someone who couldn’t quite grasp the concept of ‘keep left’ unless overtaking… all because lane one apparently wasn’t good enough.”
They added: “You really never know what you’re going to find when you stop a vehicle.”
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