The parent questioned whether the use of grain for finishing was down to a demand based on consumer taste preference.
You've done nothing that would move them from their position of questioning the evidence here.
The detail you do provide shows grain feeding increases yield for farmers, which would be an indicator that it is financial benefit to herd owners that drives the use of grain; potentially moving away from your assertion.
Angus beef is very popular in UK, I'm relatively sure it's grass fed?