Green’s Dictionary of Slang

tower bridge phr.

[Tower Bridge in London goes ‘up and down’]

a phr. used in reply to ‘How are you?’, meaning ‘up and down’, ‘so-so’.

[UK]J. Curtis There Ain’t No Justice 162: ‘How’s things?’ ‘Tower bridge,’ said Reg. ‘How’re they with you?’ ‘Up and down, same as with you.’.