But as
@tsubaki has rightly pointed out there's a lot of factors in this mate.
For one, the weather is obviously nicer so of course people are going to want to be getting out when they can for their walk or jog or whatever. In London, the only option they have for that is a park.
If they are either not working or on furlough or even working from home, people have more time. In London, a commute is probably an hour each way on average? That's two hours more each day and these people who perhaps never thought they had the time before are now using it to get out and exercise.
The kids aren't at school, or the vast majority anyway, so families are going to have to get out of the house/flat for their allotted allowed time and again, in London or most of the big cities, the only option is the parks.
Another factor - people who go to gyms now can't use them (obviously) so they are doing their exercise outside.
It's not just that people have suddenly decided they're going to rush to the park.