Straightforward to police; randomly show up at the designated isolation address several times in the 14 days. Won't take much in the way of resources.
Technically yes, it is possible. How many police get allocated? They work in pairs, they can check maybe one address in 15 minutes? Thousands of arrivals since the isolation rule came in. Not possible even to sample a small percent. This relies mainly on self control, people cannot be seen at work or out of their house by neighbours as they may get dobbed in, what a strange situation.
"Designated Isolation Address"? No such thing. On the immigration arrival card you write your "intended" address but your intentions may change. The numbers make it difficult to effectively control.
I think circumstances will overtake the isolation rule anyway, pretty soon many venues will be closed, so not much choice but to stay home. Are friends really going to gather at each other's houses as the spectre of infection grows?