For emissions, I generally put some cleaner, Cataclean, Millers.. there's an infinity of these things..., into a full tank a week or so before
the MOT. On the day, I give the car a bit of an Italian tune-up on the nearby M5. 6 or 7 miles to the next junction and back. Then into
the garage, nicely warmed up.
2006 Toyota Yaris 1.0 T3
1993 Mazda MX5 Mk1 1.6
2000 "W" HDI 110 Executive Saloon (Recycled).
The HDi is very good at maintaining low smoke all by itself, you never see and significant smoke from them out on the roads and there's rarely any complaints on here about failing MOT's on smoke.
If you'd like to see thick smoke look out for VAG, Mercedes and Vauxhalls of around the 2000's era.
Cars in my care:
2021 Kia Spottage 1.6 Pez Turbo Dual Clutch Gearbox Trickery
2013 Renner Twingo - donkey work
Oh I dunno, there were a few rolling coal up the hill at prescott
2002 110 HDi estate, Rapier in monaco blue! ( found quite a bit more power ) SOLD I've sorted the airbag light
1998 3.0 V6 SE coupe in Diablo Red ( my baby ) sold
2006 206 1.4 16v sport in Aegean blue ( wife's shopping trolley )