I'd start with a British composer. Vaughan Williams comes to mind.
Good point - you shouldn't limit yourself to one piece. Match the music to the exhibit.Parry - Jerusalem
Vaughan Williams - Tallis Fantasia
Britten - A Hymn to the Virgin
Elgar - Cockaigne
Holst - Psalms 86 & 148
Dyson / Walton - In Honour of the City
Handel - Coronation Anthems