there is no 10 mile limit in the “offical document” that seems to be “police advice”
exercise guidance
Cycling
17. Cycling is a valid form of exercise and is also a suitable way of going to work. Cycling is generally a low-risk activity but with emergency services under pressure, it is important to take steps to manage risk wherever possible. An accident or a breakdown far from home would place additional strain on health services or require a further journey to be made by someone else to provide assistance.
18. People are expected to only cycle alone or with members of their household, on routes they know well, and that are well within their ability level. Cyclists on shared paths should be considerate of walkers, runners and other people cycling: they should stay two metres from others, slow their pace and stop to let people pass as appropriate.
19. Cycling should be local, as a rule of thumb limited to travelling no further than a reasonable walking distance from home. Exercising by cycling significant distances from home is not considered to be a reasonable excuse for leaving home.
20. Cycling to work, or for work, is a reasonable excuse to be outside (so long as going to work, or doing the work, is itself justifiable).