...joining the airbus fleet at LHR at the end of Jan. I have a family wedding in May in the states
Summer DFW (Duty Free Week) bid closes Jan 9th, and Summer Leave closed ages back. You'll be fine - assuming the wedding is mid-week
May is not that "popular" for Leave/DFW... so there may be slots left available. I would consider contacting BA ASAP, or certainly within the first few days after you arrive to see what they can do...
HTH
NoD