Just as the sand eel, by varying materials, colours (and these can even be added using a permanent marker), you can imitate almost any bait fish. And while for instance sand eels aren't present in fresh water, stickle backs and many other small bair fish are (and so are trout), so the STF Fish is also great for imitation freshwater bait.
The style was invented by Morten Valeur, now owner of Ahrex Hooks (and a good friend) and it's a very popular fly in Scandinavia. It's not the easiet, I'll adit - you need to be familiar with dubbing loops, and it's not the fastests fly to tie, but doesn't take too long either. And that's outweighed by the fact that it's quite durable.
If you want to see how it's tied, here's an old video I did for a Danish magazine (then a printed one, now online only). The speak is in Danish, but if you have questions, just ask on the Board.
This light, tan version I tie in the video is the one I fish the most, but I also add brown or olive backs with a marker, sometimes dots, sometimes red gill covers, sometimes I add lots of flash etc etc. The variations are endless.
I have absolutely no doubt that this fly would be excellent on English reservoirs - I know I've caught plenty of lake trout on it myself, both browns and rainbows love it.
STF is a great material for it, it's fine, so it's excellent for flies from say 3-4cm to 6-7cm. Longer than that and I find the tail fouls a little too often using STF. For longer flies, I recommend looking for a slightly coarser dubbing, maybe a little longer in the fiber than STF.
Have a great weekend!