I don't think TT routes on open roads are allowed to go round roundabouts these days - or at least it's not recommended (elf-n-safety-gawn-maaad, innit). Our club 10 route used to involve negioting a rather horrible roundabout at the end of the dual carriageway - the roundabout is notorious as a bit of an accident blackspot and is one of the things that used to put me off doing the club 10. Now the route has changed so you turn left at the roundabout onto a minor lane, cross over the dual carriageway via a bridge and filter back onto the other side.*
Of course, if it's not safe to join the dual carriageway, you do have to wait at the give way lines - even if you're on for a PB.
Anyway, to answer your question... yes, you may have to coast a bit on the approach to a couple of junctions. But that's not what the OP was talking about - coasting on open road sections simply means you're not trying hard enough.
*In case that's not clear, this is what it looks like on Strava:
View attachment 415712
https://www.strava.com/routes/13952082