So to update this, ended up getting a paper BP due to doing an AUP on check in. (£440 WTP-CW) I asked about TSA Pre and if they'd be able to enter my known traveller number into the booking whilst in the first wing. No knowledge with the check in staff member serving me on how to do it or that it was possible.
Very interestingly I'm assuming due to the AA segment on the booking my KTN is in there... On the paper BP low and behold check the top! Not sure what good it'd do on the LHR-MSY segment
