From that date every area of England (including the territorial waters adjacent to England and the airspace above England) will be in"the Step 1 Area". A new and complex set of Regulations will apply within that Area.
From dates yet to be confirmed, areas of England (yet to be specified) will move into "the Step 2 Area". Similarly, from a further date (yet to be confirmed), areas of England will move into "the Step 3 Area".
It doesnot appear that the whole of England will necessarily move from the Step 1 Area to Step 2 etc.
Essentially, the legislation is the government's way of putting into law the "roadmap" for relaxation of coronavirus restrictions - see UK Government Roadmap out of Lockdown. The roadmap sets out the four tests to be applied by the government when deciding whether to move from one step to the next.
The new Regulations are The Health Protection (Coronavirus, Restrictions) (Steps) (England) Regulations 2021 - SI 2021/364. They comprise 94 pages of detailed legislation. The Regulations are made under powers granted to Ministers by the Public Health (Control of Disease) Act 1984 (as amended).
The Regulations also contain complicated provisions against individuals leaving the UK - see Part 3.
Part 4 empowers the Secretary of State to disapply any relevant restriction or requirement in relation to specified premises, a specified event or a specified gathering. This power is linked to research into the potential transmisson of coronavirus in controlled environments.
Part 5 deals with Enforcement and Part 6 is headed Final Provisions.
The Regulations have to be reviewed by 12 April 2021 and at least every 35 days thereafter.
The Regulations expire on 30 June 2021 but they could be continued or replaced.
There is an Explanatory Note at the end of the Regulations document but the Note is not part of the Regulations.
Cabinet Office 29 March 2021 - Coronavirus restrictions: what you can and cannot do
UK Government Press Release 5 April 2021 - Further easing of Covid restrictions confirmed from 12 April