No, don't worry, you didn't cause offence!
I just know how busy my colleague is, that she feels that she can't really say no, and so does everything she can to accommodate visits. I know that the very fact it is Heathrow means it's very tempting to want to visit. She's retiring this year, and won't be replaced (as is happening to support staff all across the company), and more and more of my colleagues are having to take on more work as support staff numbers decline, meaning they will have even less time to host visits.
At this early stage, candidates can gain as much, if not more, by visiting smaller units where the controllers themselves have more time to chat and security arrangements are more straightforward if the tower is landside.