There are a couple of separate issues here. You are allowed to sue him in Guam if he visited you there regularly, if you ever gave him money while he was in Guam, or if you can show that he reached out to you in Guam frequently in connection with borrowing the money. That's fine.
The problem is, unless he has bank accounts or owns property in Guam, you won't be able to COLLECT the judgment, which is the whole point of filing the lawsuit. A US judgment isn't enforceable in the UK. So the only way to get a judgment that you can actually use to attach his UK bank accounts, damage his credit, lien any property, and actually try to get him to pay, is to file the lawsuit in the UK.
A barrister should be able to help you prepare and file a Complaint, and you can see if the court in the area where he lives would let you file a Complaint on your own by mail. But you would have to fly to the US before trial.
Is this someone that you met in person? That you visited with and shared experiences with that weren't over the phone and online? I ask because some of what you've said sounds an awful lot like a dating scam. And if that's the case, your fiance isn't really in the UK - he's probably in Nigeria or Ghana. And in that case, it's not worth your time or money to sue him in the UK, because he's not really there and you'll never get your money back. Instead, he should be reported to the FBI as a scam artist.