its got gray runnig across the top half of its body, a black semi-circle on the about 5mm away from its head, and on its a black spot with yellow around it on its caudal fin.
Does it have a tear drop under its eye? Vertical bars above the lateral line? Kinda sounds like a Crenicichla sp. 'xingu II'...but can't really be for sure with out pictures. Your in england, so the posibilities are huge, as you can get a lot of species of pikes...