It would appear that the ready availability of water from the heavens allowed for the Belfast sink to be fitted with an overflow as opposed to the London sink which doesn't have one, possibly to save wasting water.
They're not cheap. Small sized one in Howard's link, £250, double one, more than £800.
I have two with plants in them in the garden. BUt they're so badly chipped and cracked that they probably wouldn't have been worth taking away.
Yes, they're both "Butler's" sinks. The "Belfast" has an overflow, while the "London" doesn't ................ thanks to Douglas and Howard's links, I now know why :o)