Hill Street has been available here in the US for some time. In fact, there is a box set of the entire series for sale on Amazon. My guess is there is some sort of rights hassle if the series is not available in the UK...
Some TV series are not available for a number of reasons: the source material is gone or is not in good condition, the series was produced on videotape and the quality/resolution is very dodgy, the original producers or production companies are no longer around and there is a question of who has the rights, the series may have used published music and did not secure the rights for all media (past, present, future), etc. The number of possibilities is large and usually involves lawyers and we all know how convoluted and drawn out anything involving lawyers can be...
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