I'm the author of this blog post. And I don't think the invitation was the main issue here. I did have a ticket, and I wasn't visiting any friends there who could invite me. Also I haven't been asked about the invitation letter during the main interview, I haven't even been asked why I don't have it or anything like that...

I think the main issue was my not permanent employment, here, in the UK. I just don't like the way the officer was interviewing me.