if i was you I would avoid stopping at Luton or Hemel, I drive from newport pagnell to chertsey in surrey most days and the following hotspots are best avoided in the rush hour,
M1 south from j12 to j6a (may be bad past that but I turn onto the m25 there)
M25 from the M1 to anywhere in the rush hour, especially bad from M1 to Junc 17 and around the M40 and m4 and m3 junctions
where are you coming down from? if possible its better to come down the m40 if you are coming from birmingham way.
As for the smell in Hemel, its not been to bad the last few days but tat may be down to wind direction because some days the smell of diesel is horrible on the M1