Interesting topic!!
I think most major cities are expanding all the time, although probably incorporating towns rather than other cities as you say, Andrew...
Talking of Leeds/Bradford.....I remember when Leeds/Bradford Airport was called Yeadon Airport! Not sure why it was changed to L/B, it had been called "Yeadon" for years....and isn't technically in Leeds OR Bradford!!!! (My Aunt stays in Otley, W.Yorks in the shadow of the Chevin, and in the flight path for the airport!)
Having said all that about how L/B Airport isn't in either city - Glasgow Airport isn't in Glasgow either(!!!)
My auld dad is originally from Inverness and wasn't happy when it was granted "city" status in 2001, as, in his day it was a "Royal Burgh".