The current law prohibits agencies from supplying workers to perform ‘duties normally performed by a worker who is taking part in a strike.

In responding to the rail dispute, the government has proposed to change the law prohibiting employers from using agency labour as strikebreakers. The current law prohibiting agencies from supplying workers to perform ‘duties normally performed by a worker who is taking part in a strike or other industrial action’ is to be found in the Employment Agencies Regulations 2004 (SI 2004 No 3319), regulation 7, made under the Employment Agencies Act 1973. I am assuming that what the government has in mind is a revival of the proposal to allow agency supplied strikebreakers that had been made by the Cameron government at the time of the Trade Union Act 2016. These plans were never implemented, and may have attracted some opposition from employers as well as trade unions, in the former case because of their impracticability, as well no doubt as a desire on the part of reputable agencies to avoid the controversy associated with strike breaking.

https://www.ier.org.uk/comments/agency-workers-as-strike-breakers-ilo-convention-87-and-the-eu-uk-trade-and-cooperation-agreement/