Hi pekka,
I think you'll find spearfishing for anything in a UK river is illegal (mind you things could get complicated in an estuary!) especially if you shoot anything described as a course fish (bream, pike, carp etc.) mind you I wouldnt eat them anyway! If you shot somthing like a trout or salmon you might get away with it especially if no one sees you. (although obviously im not suggesting it or anything!

) As Crisin put it I also know a man who tried similar things twice and had a night in a police cell for both (but no charges brought). A campsite on the Gower in wales was too near a nice river and the trout were too tempting, the said man was caught in full gear with a trout wandering back to the campsite by a passing bobby.

The other occasion has no excuse other than to say it was very irresponsible and became the climax of a stag night. The ornamental lake outside said stag night location where a speargun happened to be located was teeming with big fat goldfish. Nuff said, owners were none to pleased.
Please be careful, dont get caught and dont do it naked, fish bite, and so do fishermen.
Joe