Terraces or semis they are usually on the party walls (backing onto a matching stack with the neighbours'
Detached houses they are usually back-to-back down the middle of the house. In a typical four-up-four-down detached that means two chimney stacks, one up the middle of the right hand pairs of rooms, one up the middle of the left-hand pairs of rooms.
So yes - absolutely possible to have them on internal walls (they do however have to be in brick load-bearing walls. It would be very rare/unlikely to have a stack in a lathe-and-plaster wall in a Victorian house.)
Only rarely are they on external walls but it's not impossible.
In tudor houses you can have them in the middle of a room if you want!
<Added>that winky was not intentional!