There is very little to choose in size between Galaxy 747s, the latest jumbo is actually slightly bigger . What impresses most about the Galaxy is the sheer bulk & capacity. As for the runway, there are many UK aiports which are more than capable to accept a Galaxy.
The reasons the yanks choose Prestwick are many,yes the Big `ol runway but also the fact that there`s lots of space to park the buggers, not many commercial flights to work around & its cheap !! Plus the fact there are so many good golf courses in the area ! The yank airmen love to boast of playing on a genuine Scotch golf course !