At risk of being threatened by any Scousers...
La' means 'Lad', and it's what stereotypical Scousers say instead of mate, etc.
( LeightonJimbo underestimated how much hard work a PGCE is, Fri 18 Apr 2008, 11:23, archived)

i was confused
i thought it he was singing the only bit of a beatles song he knew that mccartney didn't write.
( god save the queen hardly ever original, Fri 18 Apr 2008, 11:26, archived)

hahaha
la'
( lordsaccharine my hat it has three cornets toot toot toot, Fri 18 Apr 2008, 11:38, archived)

I think McCartney wrote most of the ones that go la
or lalala or labidoobida.
(_Felix suspicious character from prehistoric caucasus, Fri 18 Apr 2008, 12:13, archived)