It depends on the nature of the bonus payment. Under the Agency Worker Regulations 2010 (AWR), an agency worker who completes the 12-week qualifying period will have the right to the same pay as if they had been recruited directly by the hirer to do the same job. Under the Regulations, 'pay' includes bonuses based on individual performance (for example, sales commissions and performance-related pay) but does not include bonuses which are not connected to an individual's performance and which reward loyalty or length of service.
If you are unsure about your entitlement, speak to your union (or an advice agency if you are not a union member).