What I’ve found says they were thrown into the sea.
http://www.thehighroad.org/archive/index.php/t-651362.html
http://www.freerepublic.com/focus/news/650257/posts
http://message.snopes.com/showthread.php?t=17760
I saw an old movie (A TWIST OF SAND) in which SMLEs are dumped into the ocean.
What a waste if they were! I know that back in the early 1970s several US Coast Guard cutters dumped tens of thousands of handguns confiscated in NYC into the ocean.
My research shows they were melted down. And I have trawled through online pages on UK war production, the Home Guard.