Just add the booking to your bookings on the BA website. For a discount OTA, you don't go near them once you have a PNR, because they're very unresponsive usually and don't really know what they're doing.

There are good OTAs and there are poor ones, but I still use them a lot for very cheap flights. OTAs have their uses, but if you can book online the main use is to provide a point of reference for a price promise guarantee claim. For a TP run I don't think I'd stray away from a direct BA booking because it's much easier to deal with BA (seriously) if things go wrong and the amount of money involved won't be insignificant.