I don't know where at LHR the incoming DL flights will end up, but the chances of it being at one of the BA terminals is virtually nil. Two hours is the absolute minimum I would allow for connecting to another airline, and in some cases that might not be enough. There are some threads on the AA forum with a lot of helpful, detailed information on the subject. If you can do carry-on luggage only, you should, because BA not only has issues with getting bags from other airlines on time, but they have one of the worst records in the business for losing them once they have them. Good luck!