Come to think of it the Euston / Marylebone Road is a 30mph. There are a regular appearance of side road's, oncoming traffic slip roads and ATS controlled crossroads for the majority of the road, but, in a performance car I can see it possible to tickle 100 at times.
When circumstances dictate I often reach 80mph down the Euston road heading West just before Great Portland Street station, but you have to hold back until you have passed the UCH turn off as the road kinks right and descends, so there is a loss of vision at the brow of the hill. Likewise Albany Street for those who know it, heading North (a straight road that runs to the right of Regents Park). I tend to straddle the white lines here though.
Both roads shown on the map:
See Map