Covid-19 case numbers continue to rise exponentially in the UK.

It is even more crucial to do everything we can to slow the rate of onset of new cases. Nonetheless, exercise remains important for our health.

The relevant piece of the legislation in England is:

The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020

Restrictions on movement

6.—(1) During the emergency period, no person may leave the place where they are living without reasonable excuse.

(2) For the purposes of paragraph (1), a reasonable excuse includes the need—

(a) to obtain basic necessities, including food and medical supplies for those in the same household (including any pets or animals in the household) or for vulnerable persons and supplies for the essential upkeep, maintenance and functioning of the household, or the household of a vulnerable person, or to obtain money, including from any business listed in Part 3 of Schedule 2;

(b) to take exercise either alone or with other members of their household;

-----
That is an abbreviated version. Here is a link to a more comprehensive specification of section 6:
legislation.gov.uk

Similar regulations apply in Scotland, where section 8, (5) (b) states:
(b) to take exercise, either alone or with other members of their household.
In Northern Ireland, section 5 (2) (b) is the same:
(b) to take exercise either alone or with other members of their household;

In Wales, the regulations state that you can leave the house no more than once a day for exercise, The relevant section [section 8] states:

(b) to take exercise, no more than once a day, either alone or with other members of the household.

-----------

Here is a link to the guidance regarding a reasonable excuse to leave the house, issued by the National Police Chiefs Council (for England):
college.police.uk
Note added 26th May: unfortunately the clear guidelines issued in April have been taken down and replaced by a more complex collection of documents.
-------------

The fact that the government accepts the importance of exercise places a responsibility on us to exercise in a way that reduces our risk of becoming a clinical case rather than increasing our risk.

Here is a review of the factors that influence the relationship between exercise and function of the immune system
fetcheveryone.com/blog/13360/2020/3#blog403926