Johnson has added now that the police will be able to take action where people are not complying with the new measures:

If you don’t follow the rules the police will have the powers to enforce them, including through fines and dispersing gatherings.

To ensure compliance with the government’s instruction to stay at home, we will immediately close all shops selling non-essential goods, including clothing and electronic stores and other premises including libraries, playgrounds and outdoor gyms, and places of worship.

We will stop all gatherings of more than two people in public – excluding people you live with.

And we’ll stop all social events, including weddings, baptisms and other ceremonies, but excluding funerals.
https://www.theguardian.com/politics/live/2020/mar/23/uk-coronavirus-live-news-latest-boris-johnson-minister-condemns-people-ignoring-two-metre-distance-rule-in-parks-as-very-selfish