The airlines is liable to a limit of XDR1131 under the Montreal convention ( assuming it was not a domestic flight ) for delayed baggage
Originally Posted by Montral Convention
Article 19 — Delay
The carrier is liable for damage occasioned by delay
in the carriage by air of passengers, baggage or cargo.
Nevertheless, the carrier shall not be liable for damage
occasioned by delay if it proves that it and its servants and
agents took all measures that could reasonably be required
to avoid the damage or that it was impossible for it or them
to take such measures.
If you buy things that are needed because of the delay ( e.g. clothes ) then the airline is liable for it.
If you are just looking for some compensation as some "goodwill gesture" , rather than reimbursement of expenses incurred , then BA can indeed say no to that