The sheriff cannot seize your shares. However, your judgment creditor could apply to court to appoint a "Receiver by way of equitable execution" over your shares. The creditor would have to satisfy the court that it has exhausted all other means of obtaining payment (e.g. using the sheriff, instalment order etc) before seeking such "equitable relief".
Generally speaking, creditors would not seek to appoint equitable receivers, as the process is expensive and most companies do not have great value. One of the many issues facing equitable receivers is the issue of shadow directorship etc.
Jim Stafford