TG,
Why?
1) Building and improving relations with Russia is in our national economic self interest.
2) Because proper decomissioning will prevent environmental catastrophe - and as a major maritime nation with fishing interests, that's in our national interest.
3) Because UK participation will reduce the risk of certain materials and technologies going where they might do us harm.
4) Because that's what you do, after winning a war, if you want to avoid another one. Not helping Germany out in 1918 soon proved counter-productive.
5) If you want to harp back to the Arctic Convoys and WWII, you might feel better that the nation we're spending £40 m to help now shed huge amounts of blood to defeat our common enemy in 1941-1945.
My Dad is a veteran of the war in the North Sea and Arctic in WW II (albeit as a flier). He then served in the RAF until 1978, so has been moulded and shaped by the Cold War. He's recently been awarded two medals by Russia in recognition of the part he played in 'defeating the Fascist aggressor' because of the Arctic Convoy protection missions he flew, and he's pretty enthusiastic about bringing the former enemy 'on side'.