Guidance is not law. [except for some odd occasions where it is]
The Regulations (ie. the law) restricting people from leaving where they’re living is http://www.legislation.gov.uk/uksi/2020/350/contents/made – read Regulation 6
The guidance produced by the government does not reflect the law – it is much more restrictive. The Court will only enforce law, not guidance, so get some proper advice before paying any fixed penalty notice. [The regime of fixed penalty notices here seems like it could turn into a money making exercise/tax on the poor and others who fail to get advice that will allow them to defend any proposed prosecution.]
You can leave your home if you have a “reasonable excuse” and a non-exhaustive list of examples is given.
Without doing things that are currently a really bad idea because they increase the risk of spreading the virus ie. holding a party, BBQ or other event with lots of people attending and being closer to one another than 2 metres or going mountaineering or somewhere else you might get into difficulties of the sort requiring a mountain rescue team to help you out of, there is a lot of scope for being outside your home.
If you need/want to exercise outdoors more than once a day, whilst following the advice about social distancing, go for it. Same if you have a child who is used to be able to be outside and needs a lot of exercise.
If you want to deliver something to a family member or friend who doesn’t meet the current definition of “vulnerable” go for it.
If you’re sharing your home with others and it’s all got a bit tense, or worse, it’s OK to go out.
Public health is important, but so is the government being honest in its publications. There’s no good reason for the government to be issuing guidance that is so much more restrictive than the legislation without acknowledging that that’s what it’s doing.
Separate to the Regulations, thinking about the health implications at present, it’s worth being cautious around activities that pose a higher risk of injury, even if they’re ones you routinely do as a household or individually. Trampolining can cause broken arms/wrists. Various DIY jobs pose the risk of injury. The risks are of getting poorer treatment because the hospitals are overwhelmed with COVID-19 patients and of catching COVID-19 in or around the hospital.