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.
Hence, I think a total block would be better than a partial block, because that can be framed as legitimately risk mitigation and would be a lot harder to attack.
That said, some pressure would be better than no pressure, so if the alternative is no block, I'd prefer a partial block.