What are you talking about?Not if you can pick it up in Manchester and ‘import’ it to Birmingham just as easily.
You were originally saying that quarantining people from overseas was pointless. So what two places within the UK with no barrier between them has to do with what you were trying to say is entirely unclear. If there were a hard border between Manchester and Birmingham my point would still hold.
If you restrict infections arriving from outside a hard border you have fewer infections within the border than you would if you allowed the infection in. How could it possibly be otherwise?