The area around the Park Tower is absolutely safe. If you wanted to walk to Soho (which is definitely do-able), you'd have to start out in the subway (the underground passage) that gets you around Hyde Park Corner into Mayfair. I've done that at all times and I think it's safe, but it might be a bit intimidating (there are usually a couple of kids camping out and bumming change down there, but they're harmless).

The Tube is right there at the Park Tower subway and it's an alternative and also completely safe, especially pre-midnight when it's still pretty busy.