The current service pattern since the pandemic is 2tph from Forest Hill to London Bridge on Southern.
The majority of services from Forest Hill are 8tph to Highbury and Islington (4tph each from West Croydon and Crystal Palace), so I’d expect most passengers commuting to Liverpool Street/Moorgate change at Whitechapel for the Elizabeth or H&C services.
Moorgate is also possible by changing at Highbury & Islington for Great Northern.
