I played at the Royal Festival Hall before it officially opened. There was a conductor/entrepreneur named Leonard Rafter who arranged for concerts to be given with a small orchestra so that visitors to the Festival of Britain who wanted to look inside the Festival Hall could judge the acoustics. I was engaged as principal bass, and we played Mozart, Haydn, Rossini etc, pieces that did not require a large orchestra.
I did not play at the opening concert, however, but played there many times subsequently.
Personally, I didn’t like the acoustics at the time, I thought they were too dry, but I can remember some improvements being made later.
I commend the volunteers who are endeavouring to transform the Royal Festival Hall. They can be reached here.