I don't think I can cotton to "mum" because the few American books I read as a youth were Berenstein Bear things, and they used "mom." But I can still appreciate "mum" (like the way Elvis Costello used it as one of his puns in "Shot With His Own Gun").
I use "mate" occasionally, but then people think I'm using a really outdated form of the word "lover"..."man" just flows easier off of the tongue. Blame it on the Dylan/Springsteen obsession.
"Fella" is definitely an old man's word for a young man (my grandfather was infamous for raising his empty cup of coffee in a restaurant and shouting out at the waiter, "Hey, fella, how about a refill?"). And "deffo"...that's just not right! Granted, "awesome" is no great shakes either, but...
I once slipped and asked someone if they had a "brolly." "What?" she asked, "Did you just ask if I had a bra?" Ah, good times...
...and after I heard Crocodiles for the first time, I was just itching for a chance to say "bagsy" in actual conversation. It never came up, sadly.
And sunshine, you wouldn't find the Pittsburgh accent/dialect funny...actually, you probably would, but you'd also likely find it indecipherable. There are entire books written about it.
If you're bored and/or so inclined, peruse this site for your own entertainment and get back to me.
http://www.pittsburghese.com/