As far as I'm aware Network Rail continue to own the freehold of the TG Jones building and the subway below. The building we constructed to replace the WH Smith branch that was inside the original station building.
The branding of TG Jones suggests to me that the owners are looking for a quick turn around for this business with more permanent branding for the stores once they have time bring in a branding team or potentially merge the store with other offerings in their portfolio (Hobbycraft and potentially Lakeland). They may well look to pull out of unprofitable sites with short leases, but I suspect that the lease from Network Rail is cheap and was renewed a few years ago (replacing on that had been in place for 50 years), possibly for a similarly long-term. Closing any branch that contains a post office would be likely to be resisted by local residents and it is likely that the inclusion of a post office increases footfall.
But the closure of TG Jones in Forest Hill could also present a useful opportunity to demolish the building and begin the reimagining of the town centre. There are a couple of obvious sites where a small post office could be moved to in Forest Hill - Betfred and Boots immediately spring to mind, or one of the smaller grocery stores could take on the sub-post office functions that are probably not too dissimilar from the TG Jones branch.
But apparently the total TG Jones business is profitable and the cost of the purchase was relatively low compared to profits. There is no reason that the owners don't just run the business as a cash cow, adding little investment and just taking a good profit every year, just closing a few branches at a time as high streets across the country gradually die.