Oh, you JET hopefuls, always finding something new to stress out about
Yes, contracting organizations can request certain nationalities (and gender). I think many towns request nationalities based on sister city relationships. For example, there was a town in my prefecture that had a very active sister-city connection with a town in Australia, and they almost always got Australian JETs (whether they were actually from that Australian sister-city didn't seem to matter). And every year it seems a few Denver JETs go to Takayama-shi (Denver's sister-city) and Yamagata-ken (Colorado's sister state).
Some COs base their requests on which JETs they liked in the past. If they had some awesome superJET from Canada, they're more likely to request someone from Canada. If they had an awful, troublesome JET who was male, they may request only female JETs. I know, it's pretty blatant stereotyping, but it's how it goes
Anyway, these requests are limited to individual towns and schools, it's not like entire prefectures are requesting only UK JETs. And like JET's placement requests, the COs maybe not necessarily get the JET they "requested".
Should you be shortlisted, I'm sure they'll put as much thought into placing you as as they do any other JET. Just don't be surprised to learn your CO requested an American.