With all due respect, but before criticizing my response to OP's original post could you actually read it? They ask if BA has its transfer desk and I provided response.
Dozens of people transit in IST on separate tickets (it is still transit) - it is perfectly doable (although more challenging during COVID) and local Customer Service agents know how to handle them.
No need to get snarky.
If
you actually read the OP's post they advised they are catching their BA flight the following day after they arrive IST on TK. Facility of a transfer desk had already been established by the time you came to the table. My point being on 2 tickets they aren't in transit according to IATA, they're arriving on one PNR and departing on another. 2 separate itineraries. They are not in transit for the purpose of the need of a transfer counter.
My specific point to the post for which you appear to find so offensive is that BA don't operate airport check-in 24/7 and typically open a physical check-in at outstations for flights to London 3 hours before departure . If there aren't any BA flights due to operate within the hours around the time the OP is due to arrive, who is going to check them in? The handling agents transfer counter may oblige with a boarding pass but that would require them to open the check in the day before it's supposed to open. For this reason it's fair to suggest it may not come to anything.
Regardless, the OP now says they're going to go landside now so this kind of negates the issue of whether or not there is a counter.
Peace and harmony ✌️