I haven’t seen any firm stats or data. There may be something on What Do They Know so do check there or ask the question. I get the impression only from having been in this forum for many years now that many who get top180 scores are often not putting Bexley schools down. They might have a superselective on their list, be going independent or just used it as a test. Some may already be in distance anyway so their being in the top 180 wouldn’t have changed their outcome from being a pass.
I found this thread
viewtopic.php?t=46493 that mentions that in around 2015 only 60 of top 180 named a Bexley school, some of them could have had other schools higher on the list or had the CAF as a backup to independent so even then not all would have ended up at a Bexley grammar.