A few points to ponder.
The road-crossing and the killing don't necessarily have to be done at the same time. The culprit could conceivably cross the roadway under cover of night, then lay low on site until it decided to kill at mid-morning.
Weasels, minks, and ferrets - "mustelids", I think they are called collectively - are much like cats and rats in the respect that they can get their entire bodies through apertures big enough to admit their heads. There doesn't have to be a noticeable breach in order for them to enter. That's how they get into chicken coops and lay waste to their inhabitants.
Depending on how large the koi are, an otter-sized culprit may be called for. Additionally, otters are not afraid to get right into the water with their prey. I once worked at a site that had a tailings-pond that was about an acre in size, and the owners had stocked it with some sort of carp to keep the algae down. Some of those carp had survived long enough to get around 3 feet long, or just a bit over, and otters sometimes had a field day fishing for them right in the water with them. I've sat on a hill above that pond and watched them at work, frolicking and hunting carp among the water-weeds growing from the bottom of the pond.
Otters also are in possession of the strong necks and dexterous hands that you mentioned in the above post.
Another culprit at that pond was a "fishing hawk", a fairly large bird of prey that would sometimes visit the pond for a meal. I've watched that hawk fly in, then hover over a single point of the pond stationary like a helicopter for several minutes, then suddenly fold his wings and dive right into the pond after some unlucky victim exposed enough of itself near enough the surface for the hawk to strike. More often than not, the hawk flew away dripping, carrying it's next meal in it's talons.
Turtles sometimes preyed on the smaller denizens of that pond, but I seriously doubt that a turtle could be the culprit in this case, in a smaller back-yard pond small enough to be contained by a pond liner. There's probably just no way it could go unseen and unnoticed in the water.
Honestly, I don't know enough about the local fauna in that part of England to make a very educated guess - I'm just stabbing in the dark here.
.
The road-crossing and the killing don't necessarily have to be done at the same time. The culprit could conceivably cross the roadway under cover of night, then lay low on site until it decided to kill at mid-morning.
Weasels, minks, and ferrets - "mustelids", I think they are called collectively - are much like cats and rats in the respect that they can get their entire bodies through apertures big enough to admit their heads. There doesn't have to be a noticeable breach in order for them to enter. That's how they get into chicken coops and lay waste to their inhabitants.
Depending on how large the koi are, an otter-sized culprit may be called for. Additionally, otters are not afraid to get right into the water with their prey. I once worked at a site that had a tailings-pond that was about an acre in size, and the owners had stocked it with some sort of carp to keep the algae down. Some of those carp had survived long enough to get around 3 feet long, or just a bit over, and otters sometimes had a field day fishing for them right in the water with them. I've sat on a hill above that pond and watched them at work, frolicking and hunting carp among the water-weeds growing from the bottom of the pond.
Otters also are in possession of the strong necks and dexterous hands that you mentioned in the above post.
Another culprit at that pond was a "fishing hawk", a fairly large bird of prey that would sometimes visit the pond for a meal. I've watched that hawk fly in, then hover over a single point of the pond stationary like a helicopter for several minutes, then suddenly fold his wings and dive right into the pond after some unlucky victim exposed enough of itself near enough the surface for the hawk to strike. More often than not, the hawk flew away dripping, carrying it's next meal in it's talons.
Turtles sometimes preyed on the smaller denizens of that pond, but I seriously doubt that a turtle could be the culprit in this case, in a smaller back-yard pond small enough to be contained by a pond liner. There's probably just no way it could go unseen and unnoticed in the water.
Honestly, I don't know enough about the local fauna in that part of England to make a very educated guess - I'm just stabbing in the dark here.
.