Checked transactions again (on web site and app) and it just shows "Tier Points Adjustment: Tier Points Reset for New Membership Year" and does not show what Lifetime TPs you had at the end of the year, just what was deducted.

It would be much cleaner if the transaction showed the TPs and a further line showing they have been adjusted (or as you suggest what your Lifetime TPs were at reset). I guess I will have to trust that BA's IT system has done the right thing (). I will keep a note of my starting Lifetime TPs for future reference...