Unlike Southwark, Lewisham publishes a map of the furthest distance offered by secondary schools last year. You can see that all on-time applicants to forest hill boys were offered a place (I think this is for Sept 2019 start). [lewisham.gov.uk]
Also, catchments do not exist for any of the local schools. In other areas there are catchments, this is where if you live within a certain fixed distance of a school you are either guaranteed or prioritised for a place over people living further away.
I think it's pretty clear that the op of the thread understands there are no fixed catchments locally. If there were, she wouldn't even be asking her question. She is just using "catchment" as most of us do as shorthand for "furthest distance offered".