1812 Overture & a Tchaikovsky Symphony at the Royal Albert Hall. I do not remember which orchestra was playing. It was 1958 or '59. The cannon's echo may still be running round in the Gods!
I was in the local C of E youth club for about 5 years and a few times a year we would go to London, usually the Albert Hall or RFH. In addition to "classical" concerts (baroque through romantic & 20th century periods) I remember Acker Bilk's Paramount Jazz Band and Monty Sunshine's "Trad Jazz". We also went to the MJQ at the RFH in (I think) 1961.