Best may be a loaded term for a list such as this. Let’s say “best” given my own interests and predilections, which tend to run to the dark, weird, and slightly kooky, as opposed to more mainstream offerings. Because while my preference of UK over US shows is obvious, there are still travesties such as The Only Way Is Essex out there; the Brits can do trashy as well (or better) than the next guy. But because the seasons (or series, as they call them) tend to be short, and they’re usually not afraid to present a show as a 6-part series and have done with it, I find that UK shows tend to be able to do more in terms of pushing characters and developing plots.
To find this stuff you’ll have to make a bit of an effort. Some have made it to Netflix, some DVD, and some you’ll just have to break the law and download if you ever want to see it (seriously, I can’t wait for the day when we all abandon network TV and demand that everything be available on demand).