In this case, this means that:
- the targets (both the emissions reduction and the date) can only be amended in accordance with the procedure in the Act (which gives Parliament the right to object) and
- the targets (and other measures in the bill) are now permissible points of argument for claimants seeking judicial review of the actions and policies of public bodies, including the central government. Various court actions have already happened over the subject, although I'm not aware of any that have been particularly successful.
A sufficiently motivated government could certainly pass new statute to repeal the existing act, but given the wide cross-party support for the original bill, and the general tenor of public opinion regarding climate change, this would burn significant political capital. A far more likely action would be for successive governments to either reduce the target decrease in emissions, increase the time to obtain them, or both, based on an argument of balancing the scientific data and a need for economic growth.