We all know the rules in regard to three lanes (Do not use outside unless directed by PC Plod). Whats the rules when Motorways goes from 3 to 2, for example M1/M6 Junction 19 of M1 travelling south. Inside lane becomes A14 then you have two lanes left for M1 until the M6 joins 1 mile furher on then it goes back to 3 lanes. Can you overtake, is it treated at a dual carriageway?
yes
simon
yes
yes …y
yes…
Yes.
Yes
No
Yes 
I might as well say YES as everybody else has. 
no never ever ever
oh alright then… yes 
To the best of my knowledge, once you see the destination markings appear. i.e. short white lines with short spacings (Interestingly - not mentioned in the Highway Code), then you are on separate roads and can use the offside lane because effectively it is Lane 2 of the continuing route.
Similar examples are M5 N/B @ M42, M5 N/B at Ray Hall. M42 S/B @ M40, M5 S/B @ M4.
Yes
Sorry, what was the question 
Magic Mel:
We all know the rules in regard to three lanes (Do not use outside unless directed by PC Plod). Whats the rules when Motorways goes from 3 to 2, for example M1/M6 Junction 19 of M1 travelling south. Inside lane becomes A14 then you have two lanes left for M1 until the M6 joins 1 mile furher on then it goes back to 3 lanes. Can you overtake, is it treated at a dual carriageway?
Yes you can overtake but it’s not classed as a dual carridgeway, it’s classed a a two lane motorway as it still has a hard shoulder. If it was classed as a dual track you would have to drop your speed to 50mph aswell.
You can use the out side lane on a three lane motorway when you enter a split for the motorway: example M40 North splits into M42 North and West, once under the gantries defining inside lane North and 2nd + 3rd lane West as it is classed as two different motorways (directions), but like someone else has said you must wait untill it is clearly defined that the lanes have changed i.e. road markings and signs.
Drive Safe