There are more than enough First trained crew, probably too many and it’s BAs roster and management shortfalls at play here.
If I had noticed that there wasn’t enough First trained crew I would have highlighted this but could just be told that there isn’t any suitable crew available.
If in the preflight briefing I had to ask a non trained crewmember to volunteer I would have given them the most support I could offer, I don’t however think the customer should be made aware in advance of any service issues.