Several very happy D 20 owners posted over on UMGF recently in the Guitar Critics section.
If I were the authorized UK distributor for Guild, I think I'd take a negative view of the situation. They will probably take a that wasn't bought from us, it's not our problem view of things. My local Martin dealer does not do Martin warranty work if you didn't buy the guitar from them. Entirely justified IMHO, for they know many have tried out guitars there, then shopped for the best price, then expect the best repair shop around to do the warranty work. I, on the other hand, walked my Martin with the binding issue in and received first class treatment.
I don't think even Martin has close to the same warranty out of the US. While I can sympathize with your plight, the OP has 11 posts. One could speculate, wrongly or not, OP joined just to trash Guild, wrongly or not. In true LTG fashion, those with 2,000-15,000 posts are trying to be helpful. Subjecting a guitar to an airline cargo hold in anything less than a flight case is fraught with danger. I suspect the OP will be presented with several unsatisfactory options.
I once had a friend who lived in the UK for a while who on returning home, bought a D 28 to take back. It paid for his airfare. He returned with a cheap pawn shop guitar, so he had a guitar coming and going. Dealt in cash. Everyone happy but the tax man.