So basically, they originally had these things locked up in a research site in North Carolina, but the Not-Deer kept breaking in by digging tunnels under the site and breaking their buddies out, like they're the goddamn Sinaloa Cartel. So instead of fortifying the site against deer trying to tunnel in, or putting them in a trailer truck and transporting them to a US site outside of Appalachia, they decide "oh, let's send them all to Engerland instead."
Air travel is expensive. You have to pay for the plane. The fuel. The pilots. Armed guards to shuffle the Not-Deer on and off the plane without killing everybody. The fortifications necessary to prevent these guys from breaking out of the cargo hold and opening the airlock mid-flight, because that sounds like something they'd do. At least two private airbases, because you can't risk the civvies seeing this. Land transportation to and from the research sites and the airbases, and everything that entails. And
then instead of putting them on a flight to Puerto Rico, or Hawaii, or Guam, or anywhere off-continent that's still under US jurisdiction, you drag the Brits into it and make it an international issue.
So let's say you work for the Foundation's accounting division. Which flight are
you going to sign off on?
Oh, and that's not even the best part.
Eventually, the Not-Deer break out of the site in Essex too, so now England has an uncontained population of these things running amok. Great planning, guys.