This site gives you the ability to create a circular training route within your set distance, with the option of different options on terrain etc. However, the bad news is that this function is only available for the US at the moment, but IMHO its worth keeping an eye on the site for when the UK is included.
That said, if you select the "Walk/Run" travel mode, then it comes up with a number of very viable route options for a bike.
To me its a great idea as it would take you away from the lanes/routes that you keep riding over and over.
