Note that I haven't seen any claim from the UK government that the existing rules aren't sufficient as long as they're being followed - a claim which I might find hard to believe but at least would be consistent with threatening a broader ban on outdoor activity. The quote from Matt Hancock is: "If you don't want us to have to take the step to ban exercise of all forms outside of your own home, then you've got to follow the rules." As though we're all school children being told that we'll all get detention if the trouble makers in the class keep acting up.
It feels like the purpose of the lockdown - to reduce transmission rate of the virus - is being forgotten and there is increasing focus on having a list of Allowed Activities and Forbidden Activities, rather than trying to make a consistent trade-off between cost (risk of transmission) vs benefit (health - including, very importantly, mental health effects of retaining or losing freedom, not to mention the more ideological desire to keep some semblance of civil liberties).