Your NOT allowed (officially) to undertake in the 1st lane as it's classed as dangerous driving i.s if the person in the right hand lane was to pull beck into the 1st lane & hit your car YOU would be to blame (yes i know it's shit but it's just another stupid rule we have in this country)!!

Your supposed to stay slightly behind the car in the overtaking lane even if he's doing 30mph in a 40

HTH