It is based on your normal weekly remuneration at the time of redundancy.
For those who have had their hours reduced, the general rule is that unless the employee has agreed to a permanent reduction, that redundancy will be based on the full-time hours.
For a reduction in the hourly rate, the same principle applies. However, the employee would have to be in a position to say that the reduction in pay was a temporary measure. If it was not a temporary measure, then redundancy would be based on the lower rate. Most cuts in salary are considered to be permanent, so its likely that it will be calculated at the reducted rate. However, this has not been challenged in the courts yet AFAIK, it may be worth putting a claim in to the Employement Appeals Tribunal for the difference.
Or better still, negotiating with the employer to pay at the higher rate! Talk to your Union. If you're not a member of a Union, then join one!