Well, there are seven species of pike (one of which is the muskie). The muskie is indigenous to the United States so is unlikely (while you could transport the fish and introduce it as an invasive species, why would you? Also, customs would be all over you.) Chain, Redfin, and Grass pickerels are all indigenous to North America as well, and the Amur pike are from East Asia but does have an introduced population... In Pennsylvania. So while the Amur might be a maybe in some manner, let's consider all of those species off of the table.
This leaves (in terms of known species) for Europe the Aquitanian pikes of France; the Southern pikes of France, Italy, Switzerland, and potentially the Balkans; and the Northern pikes which absolutely appear in Britain (as well as Ireland and on the North American continent.) These Through the utilization of Occam's Razor, the Northern pikes are certainly your best bet. It's possible to be a new species of pike but there's nothing to indicate that at all. With that in mind though, Northern pike and Muskies can hybridize so maybe others could, to create a hybrid species in Europe. But again, no evidence to actually support this based on just a fish corpse.
I do know over here in Alaska, up in the town of Minto that the pikes here (I assume that they are Northern pike) can grow up to four feet in length. They won't fit in your cooler. I doubt that's their absolute upper limit so a meter and a half isn't unreasonable for their size.