See Section 32B of the Animal Health Act. Annex C and other parts of DEFRA's "Contingency Plan" 3.1 fulfill the criteria needed to confer legality on pre-emptive "firebreak" slaughter - should the Secretary of State wish to exercise those powers.
Extract from the Animal Health Act 2002
"32B Disease control (slaughter) protocol
(1) This section applies to a power exercisable by the Secretary of State under-
(a) paragraph 3(1)(c) of Schedule 3;
(b) such other provision of that Schedule (as amended by an order under section 32A(1)(a)) as the Secretary of State by order specifies;
(c) such other provisions of this Act relating to the control of disease as the Secretary of State by order specifies.
(2) The Secretary of State must prepare a document (the disease control (slaughter) protocol) indicating-
(a) the purposes for which any power to which this section applies will be exercised;
(b) the principal factors to be taken into account in deciding whether to exercise the power;
(c) the procedure to be followed in deciding whether in any circumstances or description of circumstances the power is to be exercised, which shall include the application of such methods of detecting disease in animals as may be available;
(d) the procedure to be followed by persons who have functions in relation to the exercise of the power;
(e) the means by which a particular decision to exercise the power may be reviewed.
(3) After preparing a draft of the disease control (slaughter) protocol the Secretary of State-
(a) must send a copy of the draft to such persons and organisations as he thinks are representative of those having an interest in the exercise of the power;
(b) must consider any representations made to him about the draft by such persons and organisations;
(c) may amend the draft accordingly.
(4) After the Secretary of State has proceeded under subsection (3) he must publish the protocol in such manner as he thinks appropriate.
(5) The Secretary of State must from time to time review the protocol and if he thinks it appropriate revise the protocol.
(6) Subsections (2) to (4) apply to a revision of the protocol as they apply to its preparation.
(7) The power to make an order must be exercised by statutory instrument subject to annulment in pursuance of a resolution of either House of Parliament.
(8) It is immaterial that anything done for the purposes of subsections (2) to (4) is done before the passing of the Animal Health Act 2002