Possible Clues:

• Last call to raise spirits at Number 10
• Londoner's ''Bye!''
• Parting word to encourage 10
• Yippee-i-o, I'm off!
• Applaud ten for one drink
• It may be said when 15
• Before a drink there might be three such, followed by ten more
• The man with anger retired in company farewell
• European port sustaining guerrilla leader for so long
• 'Farewell'