The fastest I've done is around 45mph, going down the
Cirencester Road into Cheltenham. At that time, maximum reading cycle computers weren't common, so I only got the speed by looking at the reading whilst doing it, which is not recommended! That was also on a tourer, so the limit on speed was largely due to the fact that I couldn't put any more force into the pedals, as I was turning them as fast as I could on something like 48x12 (28/38/48 triple, and 12-24 6-speed block iirc).
That hill was relatively long, straight, and has no turnings or junctions for a while, so you can (more or less) safely get quite a speed up. Mostly I don't dare do these sort of speeds, since some idiot at a junction, who doesn't believe cyclists go faster than 15mph, may try and pull out in front of you.
I managed 28.6 mph on the flat, with straight handlebars last week, going along
Queenstown Road, but that doesn't have quite as many contour lines as Cirencester Road (it's damned near flat).