If they can identify individual human faces through algorithms, they (= technology companies with deep pockets) can surely recognize most porn pictures, given the right incentive (regulation.)
But.. I think the problem is more a social one. Is Google going to comply with a single country's law when it requires quite an elaborate technological solution? The UK is not at the scale of China, and I suspect such companies will generally ignore the law (it's common in the UK to have laws which are broadly ignored, they're on the books for selective enforcement at a convenient time) or block the UK entirely.