It depends on one's reasons.
Exercise allowed (once per day iirc).
Going to work allowed.
Shopping allowed, preferably for food, pharmacy, and nothing else. But many shop types were still allowed to be open, including Post Offices.
All 'if necessary'. I.e. The overwhelming aim was to get people to use common sense and minimise their outings. To be honest, it worked reasonably well.
As to how it has now changed as of Monday... or Wednesday... er... to be honest it's not a lot different in any substantive way afaict. Afaict and afaicr, the only big change now is (a) more instances of exercise are supposedly allowed and (b) that the government is clearly very, very desperate to get the economy going again and so are now strongly encouraging people to go to work if it is safe to do so, whereas before they were saying "only go out to work if you if you really have to" or words to that effect. Oh, and some open-air garden centres are being allowed to re-open apparently. Of course, the hospitality industry is still screwed by state mandate and a vast proportion of it will be going bust now, no matter what.
It is all delicious British and vague.
Short answer (then and now): Do what you think you have to, use your own common sense, and don't cough anywhere in public, otherwise we'll end up with a scene from
Invasion of the Body Snatchers...