Was working on my '74 UO8 this am and should have measured the dropout although with such an old bike, who knows what re-spacing may have been done.
Fitted a 6 speed wheel with 126mm spacing and had to spring the dropouts just the tiniest bit so would guess it was 124 - 125 mm back there.
The long stays on a UO8 do lend themselves to widening quite well and the frame tubes are not as stiff as some other bikes.