A bob is a shilling which equals twelve old pence. There were twenty shillings to the pound. Old money hasn't been used in the UK since, I think 1972 when we went decimal.
Another WW1 ditty that I particularly enjoy was:
I don't want to join the army,
I don't want to go to war.
Oh, I'd rather stay at home,
And from there no longer roam.
Living off the earnings of a high class whore.
Sentiments I'm sure that many of us can empathise with.
More of the old songs can be found here, some use the F-bomb.
http://faculty.buffalostate.edu/fish...ongs/les01.htm