Possible Clues:

• My word, how Ira's lyrics were set
• As were Daniel Deronda and The Road to Wigan Pier — gosh!
• 'I say!'
• Exclamation that describes 13-, 20-, 30-, 39- and 51-Across
• Best saying 12 at start? That's unexpected!
• Fancy times with last of people in Cheddar perhaps!
• Astonished exclamation
• My word!
• Words from Professor Higgins, and where you’ll find the starts of 17-, 26-, 35- and 48-Across
• Singer's Round 1 elimination — that's a surprise