These are some of the US YA/MG fantasy novels:
The Mortal Instruments Series by Cassandra Clare
Twilight by Stephenie Meyer
Wicked Lovely by Melissa Marr
Iron Fey series by Julie Kagawa
These are some of the UK YA/MG fantasy novels:
Harry Potter Series by J.K. Rowling
Bartimaeus Series by Jonathan Stroud
Tunnels series by Roderick Gordon & Brian Williams
The Crowfield Curse by Pat Walsh
Now, lets discuss one by one.
US: Usually use real person
UK: Usually abstract, drawing, items, or anything but real person.
2. Main Character
US: Not all, but most of main characters YA/MG in US fantasy novels are girls. Except for some like Eragon, most famous YA/MG fantasy novels from US feature girls as main character. And usually it was written in first person POV. I don't have the number, I don't know if anyone had research about this before, but I think it must be more than 70% YA/MG US fantasy out there written in first person POV. Not only that, most main characters in US fantasy are super handsome or beautiful. Basically appearance matter, which prone to be Mary-Sue and Gary Stu.
UK: UK YA/MG fantasy novels usually feature boys as their main characters and written in third person POV. Some combine first person and third person (ex: Bartimaeus trilogy) but still the amount of fantasy novels written in third person POV are bigger than first person POV. UK Fantasy also considered appearance as important, but instead making it handsome and pretty, usually they made it unique.
3. Plot Style
US: The common theme in US YA/MG Fantasy is teen angst and love triangle. Love story became the important thing, sometimes as important as the action or the good vs bad plot.
UK: Most of UK YA/MG Fantasy features coming-of-age story. Most notably, Harry Potter. Love story usually only featured as side-story, hinted, or even not exist at all.
I'm not going say one is better than another. I think it all depend on what kind of fantasy story do you like. Personally I like third person POV more, I also like coming-of-age theme and I prefer love story as side story, which is why I tend to read and like UK fantasy more. But it doesn't means I didn't like US Fantasy because some of them were great and I enjoyed it very well.
So, did you spot any difference I haven't listed in here?