Your premise is a little off, which changes the question somewhat.
The actual clause in the 2016 Junior Doctors contract Section 3 (52) states:
Where a doctor intends to undertake hours of paid work as a locum, additional to the
hours set out in the work schedule, the doctor must initially offer such additional hours of
work to the service of the NHS via an NHS staff bank of their choosing. The requirement
to offer such service is limited to work commensurate with the grade and competencies
of the doctor rather than work at a lower grade than the doctor is currently employed to
work at. Additional work, such as; event and expedition medicine, work for medical
charities, non-profits, humanitarian and similar organisations, or sports and exercise
medicine do not fall under the scope of additional work as a locum.
(note that 'locum' has a standard definition of 'a person who stands in temporarily for someone else of the same profession, especially a cleric or doctor' but is not defined in the contract itself - its a well known term that does not require further definition, but it plays an important part in your question)
So, a few things from that section:
Where a doctor intends to undertake hours of paid work as a locum
This immediately sets limits on what the clause means - the intention is to limit locum work, not general work.
The requirement to offer such service is limited to work commensurate with the grade and competencies of the doctor rather than work at a lower grade than the doctor is currently employed to work at
A FY2 or later junior doctor can go elsewhere to work as a house officer if they cannot find work as a senior house officer, again restricting the clause to similar working conditions and not general work.
This is very very similar to any company putting a non-compete in their contract for, say, a software developer - you need the companies permission to go do contract work for another software company for example.
To specifically talk to your point about being a gardener, this clause does not cover that because the employee is not acting as a locum in their professional capacity at that point.
The intention of this clause is to fill the gaps in the NHS staffing plan caused by 11 years of mismanagement and underfunding - no longer can an in-contract doctor or nurse work their 38 hour rota and then go fill a lucrative shift with a private hospital, they must offer any hours under 48 to the NHS via the employee bank - this means the bank can pay minimal wages to cover shifts it had to pay locums a lot more to cover before 2016.