The writ is not valid. You can apply for it to be set aside and ask for your costs.
Article 8 of the High Court and County Courts Jurisdiction Order 1991 states:
Enforcement
8.(1) A judgment or order of a county court for the payment of a sum of money which it is sought to enforce wholly or partially by execution against goods:
(a) shall be enforced only in the High Court where the sum which it is sought to enforce is £5,000 or more and the proceedings in which the judgment or order was obtained did not arise out of an agreement regulated by the Consumer Credit Act 1974;
(b) shall be enforced only in a county court where the sum which it is sought to enforce is less than £2,000;
(c) in any other case may be enforced in either the High Court or a county court.
(2) Section 85(1) of the County Courts Act 1984 is amended by the insertion, at the beginning of the subsection, of the words "Subject to article 8 of the High Court and County Courts Jurisdiction Order 1991,"
Paragraph 8 of the High Court and County Courts Jurisdiction (Amendment) Order 1999 states:
8. In sub-paragraph (1)(b) of article 8, for "£1,000", substitute "£600".
Quoting the above-mentioned legislation, you apply for a stay of execution of the writ because the transfer up to the High Court for enforcement is not compliant with the operation of Article 8 of the High Court and County Courts Jurisdiction Order 1991. Here is how to apply to stay the writ.