I regularly make a right turn across the Baker's Arms dual lane roundabout near Poole, Dorset, which is a fast A road joining a fast dual carriage way. I take up position exactly as if I was a car in the centre of the right hand entry lane, but it usually requires a full blown standing sprint (while working up the gears) into the inner lane for at least a while, then check left, signal left, move to the left lane, exit, all the while making sure the traffic entering the roundabout has seen me. It's one of the reasons I moved to combined gear and brakes - ie shifters on the handlebars. Changing gears on the bar ends or down tube isn't fast enough. If I couldn't make about 20 mph on the roundabout I wouldn't attempt to cross it by bike, Getting off may be the safe option!