This is quality television as one would expect if you respect production companies. It is true the first series has dangerous similarities to The Bridge, in my case, the American version was the one I had seen, though I did try to watch the other. Regardless, that first series of The Tunnel was far from anything bad, the lead girl was engaging, the male lead was easy going, and both used authority with class.
The Tunnel - Sabotage, the second series, strikes a far more confident stride and would have got a 10 out of 10 from me if it was stand alone. It certainly can easily be watched as that, stand alone, and this is what I recommend. You don't need to watch the first series for Sabotage. It has a thoroughly interesting, plausible yet riveting plot, is full of perfect performances from perfectly cast characters.
Funnily enough I also recommend The Bridge (The American version) series one, but less so series two, the first getting my 10.
Reviewers should see the positive side of exploring a similar theme by several companies. If we were talking about series about zombies no-one would raise an eyebrow.
If you like British or European crime then The Tunnel - Sabotage is your perfect viewing.