I found a more up to date reference from 2010:
http://www.wrecksite.eu/wreck.aspx?136939#119668
It appears the Poseidon was scrapped in China, but they claim that there were no remains to be discovered, but ALSO that there records are a little 'lacking' from those years.
PS. Rememeber the U-boat (U-534) that was raised in the north sea back in the early 90s, it was all over the BBC? It was on display largely intact in Liverpool, however the museum went under and the U-boat was sold.... to move it they chopped it into three! Apparently it is now displayed in its three sections in Liverpool. I suppose it is better to do that than leave it to rot, but all the same....cutting it up, it doesn't sit right.
PPS. The british government attitude to wartime preservation used to stink! Just look down the East coast of the US, they have got a vast amount of preserved ships and submarines.... and us.... one 'wartime' sub in portsmouth, a Cruiser (Belfast) and possibly a destroyer. I've always hoped that the Government would buy or trade back HMS Hermes from the Indians when she gets decommissioned and dock her in the Thames. Perfect ship: Wartime design, Cold War service, sent to the Falklands in 82, all bases covered.