The other usual places are the Royal Festival Hall for standard large-orchestra concerts and the Queen Elizabeth Hall, next door, for chamber orchestras. Cadogan Hall is a newish addition: mixed bag of programming including some orchestral concerts. During summer the Proms are at the Albert Hall and those are very largely orchestral, but outside that 8-week season there aren't many concerts there. There's the Barbican, as you already know. Milton Court, attached to the Guildhall School, has some interesting orchestral concerts, normally on a smaller scale, as does LSO St Luke's. Student performances, normally amazingly good, at the Royal College of Music in South Kensington and the Royal Academy of Music in Marylebone, as well as at the Guildhall.