Can a single country trip be a trip of a lifetime? I think it can, but I don't think I've ever been on a trip like that. For example, if you spent a month or more traveling all over China, that could easily top the list. But I don't think spending four days in the UK would count. It's just not quite monumental enough.
Then again, I guess you could spend an extended amount of time in one place and that could be a trip of a lifetime, depending on what you did while you were there. For example, spending two and a half weeks on a remote private beach in the Maldives might make a trip of a lifetime, mostly because you probably won't ever do that again and it's amazingly beautiful.
Sometimes you might qualify something as a trip of a lifetime based on what you did. For instance, if you went somewhere and did an intense amount of hiking, kayaking, rock climbing, and cliff jumping, that might be a trip of a lifetime. I think often times beautiful places with exciting activities make quite memorable, unable-to-be-replicated trips.
As anyone who reads this blog would know, I have done a lot of traveling, but I'm not sure that I've yet found my "trip of a lifetime". I guess I'm just too hesitant to label one trip like that and then change it later when I go somewhere and do something better. But then that begs the questions: Can you have more than one trip of a lifetime? and, Can my trip of a lifetime change over time? I think the answers are yes and yes. I think a person could have several trips of a lifetime, but if someone tells me they have ten or more, I would probably question their standards. I also think that your trip or trips of a lifetime should be changing over time as you go to new places and experience new things.
For me, I feel like I'm just too young and have too much more traveling to do to tell you what my "trip of a lifetime" is yet.