Nothing, but technically Lemmings is not owned by Psygnosis since Psygnosis does not exist anymore. Just wanted to correct the statement, that's all. hence the "By the way".
it would be Sony, since they have periodically re-released in on each iteration of Playstation & PSP over the years.
But as the previous poster mentioned, it would be easy for either of them to jump on such versions and ask for them to be withdrawn if they wanted. Much money has already been made on Lemmings, it's not sold anymore anywhere, there are no sequels planned either, so it's pretty much "abandonware" while it's not official.
If there's a lawyer around he can maybe provide perspective ?