Boss and CO, considered one of the finest British gunmakers, did and still do offer U/O rifles in calibres upto 600 NE. And they charge more for them than they do for their SxS guns.

Historically, Lang of London, Lancaster, Dickson and several others experimented with side opening guns in the U/O configuration to avoid the deep opening required on traditional U/Os but they switched to the conventional format because side opening guns were awkward to reload especially for someone used to conventional guns. Pollard's History of Firearms has a detailed section on these experiments.

Good hunting, everyone!