Asking from a position of ignorance, where is Southern doing badly? Services have been cut to match the fleet size, which was a near 20% reduction. There don't appear to be more cancellations.
ORR produces quarterly figures, and Oct-Dec are being published this coming week. But using latest (per link) let's put some hard numbers on this.
As there is talk of transfe some units from Govia Thameslink Railway to South Eastern, quoting both figures for reference
Passenger journeys (table 1) GTR 69.3m, SouthEast 31.4m
Passenger vehicle Km (table 5.1) GTR 108.0m, SE 59.5m
Train Km (table 4.1) GTR 13.2m SE 7.7m
simply dividing vehicle Km by train Km gives average train length
Then using the pre pandemic July-Sep 2019 from ORR tables 1223, 1253, 1243 respectively
Passenger journeys GTR 89.9m, SE 45.3m
Passenger Vehicle km GTR 142.9m, SE 66.8m
Train km GTR 17.8m, SE 9.1m
So for Thameslink passengers are at 77%, vehicle km 76%
For South Eastern passengers are at 69%, vehicle km 69%
Unfotunately there is no breakdown between Great Northern, Southern and Thameslink
Here are the Oct-Dec 2019 figures so can compare when new figures come out this week
Passenger journeys GTR 92.5m, SE 47.3m
passenger vehicle Km 139.0m, SE 64.2m
Train Km GTR 17.2m, SE 8.8m