I am usually one to say you should use caution and straight thinking before making any assumptions--but in this particular case, I think you are fvcked.
The thing is, you've been together a LONG time. She is showing signs of getting antsy (her meeting this guy and cultivating a so-called "friendship" with him). Personally, I think she has one foot out the door. She may not even know it herself. But deep down psychologically, she is preparing to get herself fvcked by this brit. Even if it's not him, it'd be someone else. The point is she's bored and wants to experience other people.
Even if you were to confront her on it to get it out in the open, she would deny it. Like I said, her antsy-ness is deeper than that. I'm sure you've felt that stirring of your loins before, telling you you need to be single again and enjoy fvcking other women. She's feeling the female equivalent of that.
I've been on overseas vacations. The whole POINT of overseas vacations is to fvck other people. It's so easy you can't even imagine. If you've been overseas, you know what I'm talking about. It's all about the excitement. Women are so easy when they're out of their element--when they're out on an adventure, because sex is part of that adventure. That's why people always say it's better to be single when you're travelling.
My first gf, back when I was an afc, went on a European vacation without me. I've posted here on this board about it many times. When she got back, she was a different person. Cold, distanced. Your girl will most likely be like that when she gets back from her trip. Basically, even though my gf wouldn't admit it, I suspect she messed around with other guys on her trip. When she got back, the relationship was dead in the water. I was forced to end it, but that was only because she, being a female, was too chicken to end it herself. That was the one and only time I suspected a gf of cheating.
Since then, my gfs have taken vacations with their gfs and I know they didn't cheat. I can't explain it, but you just KNOW when you have your girl hooked. It might be about me having a different kind of confidence than I did when I was in college. My gf knows if she were to cheat on me, I would smell it and it would be over instantly. That threat of losing the best keeps her in line. I really can't explain exactly how you reach this point of control over the relationship, but I think it comes with experience and being a certain type of man.
For the record, I think your girl is laying the ground work to fvck around on this trip. It's hard for me to say exactly how you should handle it since I don't know the dynamic of your relationship. Personally, I would write her off and get ready to see other people.
Good luck.
Jake