Yep that's correct, used that quite a bit back in the day. Used to cross from the slows to the fasts south of Clapham (at Balham I think), then called at East Croydon and Gatwick Airport only after that. On the northern side of town it was Rugby / Northampton / MK / Watford Jcn. only, before heading off west at Willesden down towards Clapham.
Not only that, but they crossed on the flat at Willesden West London Junction and used the fast lines to the other side of Watford Junction (after calling there) eg crossing to the slow lines at Ledburn Junction. The up/down low level goods lines weren't available for passenger trains at the time, although they have since been upgraded (by January
2021 2001) to allow today's trains to go underneath the main lines before joining the slow lines later.
For example, 1M40 13:56 Brighton to Rugby (September 2000 timetable) which didn't use the slow lines at all until it crossed at Hanslope Junction (this one had to wait to slot in behind 1B03 15:20 Euston-Northampton at Willesden West London Junction):
