You want to be looking at routes where you can take advantage of long straightish A or B roads through the night. There's some long empty A roads in Suffolk that head north and you could use to get yourself started. I know that if you go north through the fens you can rack up 300km in about 11-12 hours, even without storm winds, and reach the Humber bridge but return will likely be somewhat slower. So maybe aim for Lincoln, as a turn around point, and a total of 500km for the 24 hours.