I don't think it's as simple as that, paragraphs 533-536 of the Tier 2 & 5 guidance seems to suggest that self-employment is permitted - although only in limited circumstances.
MIGRANTS WORKING ON A CONTRACT BASIS
533. Where a migrant is working on a contract basis and has been supplied to you by another organisation, you must be the organisation which has full responsibility for determining the duties, functions and outcomes, or outputs of the job the migrant is doing.
534. An example of this would be where Company A has a contract with Company Z to deliver a piece of work. A migrant who is sponsored by Company A, may be sent to work for the duration of the contract with Company Z, but they remain employed by Company A throughout the period of the contract and Company A is fully responsible for deciding the migrant’s duties, functions, outputs or outcomes. In this example, Company A remains as the migrant’s sponsor.
535. You are only allowed to assign a CoS if it is clear that you have full responsibility for deciding the duties, functions and outcomes or outputs of the job. Where the sponsored migrant is carrying out work for a third party on your behalf, they must be contracted by you to provide a time-bound service or deliver a time-bound project on your behalf. They must not be contracted as agency workers, or to undertake a routine role for the third party, which
is not related to the delivery of a time-bound service by you.
536. If the migrant is working on a
self employed basis, there must be a contract for employment/services between you and the migrant. This contract must clearly show;
a) the names and signatures of all parties involved (normally, this will only be you and the migrant); and
b) the start and end dates of the contract; and
c) details of the job, or piece of work that the migrant has been contracted to do; and
d) an indication of how much the migrant will be paid.
http://www.ukba.homeoffice.gov.uk/sitec ... sguidance/
So you still can't go off contracting yourself all over the place without getting sponsored each and every time - including getting a new visa each time.