To the best of my knowledge, these are the main two sections:
Social Security Administration Act 1992 (s187):
187.—(1) Subject to the provisions of this Act, every assignment of or charge on—
(a) benefit as defined in section 122 of the Contributions and Benefits Act;
(b)any income-related benefit; or
and every agreement to assign or charge such benefit shall be void; and, on the bankruptcy of a beneficiary, such benefit shall not pass to any trustee or other person acting on behalf of his creditors.
Tax Credit Act 2002, Part 2, Section 45 'Inalienability'
(1)Every assignment of or charge on a tax credit, and every agreement to assign or charge a tax credit, is void, and, on the bankruptcy of a person entitled to a tax credit, the entitlement to the tax credit does not pass to any trustee or other person acting behalf of his creditors.
I believe Barclays tried to defend on these points by stating they only applied to 'legal charges' but I've not heard an update on whether it was a successful defence or not yet - personally I doubt it.