They are free to do that, but what of it? Sanctions and boycotts are "performative" in the same sense, and yet they continue being a popular tool to compel voters and politicians of other countries to act or refrain from acting in particular ways.
Wikipedia is a popular website that many people depend upon; denying access to UK users would not only create a massive inconvenience along with the temptation that it could be avoided if the law were rolled back, but would also encourage more UK users to adopt VPNs, which would subvert the law's effectivity along with that of a plethora of other authoritarian measures that the UK has in place.