I don't know what I can do around the flatlands of cambridgeshire but in more hilly west suffolk my rides into essex and central east suffolk, some 80-100 miles, average around 15-16 mph depending on wind conditons. On some shorter rides I have managed averages of 16-17 mph. Any speeds over 20 mph are difficult for me to sustain for any length of time just due to the rate my legs are spinning. I have managed 23-32 mph over 3-4 miles before but that was exceptional for me and my legs span at 32 mph like washing in a washing machine.
I believe I have 42-16 gearing on 700c wheels. My fitness is the main issue.