I now have a marriage certificate for George WHITTLE and Lavinia SMITH (1889). He says he's 22 years old, (ie. born about 1867) which is about right for being the son of George WHITTLE, boatman, and Emma ALLOWAY.
His occupation is 'Carman' (ie. a jobbing cart driver). His father is George WHITTLE, occupation 'Carter'. I believe a carter could be transporting goods either by road or water, but I am not sure about that.
Lavinia's father is William SMITH, 'Traveller'.
They were both living at 21, Wellesley Street, Holborn, London, and the witnesses were W. Grafton and M.A. Grafton - no idea who they could be.
So the certificate is not as conclusive as I had hoped, though the dates tie up nicely.

I think the 1891 census that Deb transcribed shows that, even if he wasn't in the fairground business when he got married, he soon teamed up with Lavinia's family, as we thought.
More thinks needed now...

Best wishes,
Edwina