I can't speak much to British opinion. I know there was widespread opposition to slavery in principle, but roughly from when the UK abolishes slavery in the 1830s (India excepted) there's also a sense that it was a mistake. Abolition was sold on the theory that the British West Indies would keep on producing as before, or do better. Turns you that you can't drive free workers like you can slaves and lots of slaves would rather just grow food for their families rather than crash crops for export. To that set, abolition is a humanitarian folly. This was appreciated in the US, where the UK's efforts to restrict the slave trade and promote freedom were seen by white southerners as a way to drag everyone else down to their level.

There were some regulations adopted by state governments on how slaves should be treated and they came with punishment for violation. But we shouldn't understand them as modern labor laws enforced by a vigilant state. Slave state governments were barely ever vigilant about anything except preserving slavery. An active state was understood as one that could be turned to antislavery purposes, an attitude which carries over heavily into national politics.

I'm not familiar with all the details, but typical slave codes barely considered the welfare of the enslaved. They make some provisions toward it, but they're insubstantial and ineffective. You can do almost anything you want to an enslaved person, short of murder, and it would not be a crime. Murder is, but it's virtually never prosecuted; I think there are less than five cases in the entire antebellum. I've read language to the effect that if moderate correction ends in the death of a slave, it's fine. Your jurors would be all white men in a slave society, so their idea of moderate is going to be exceptionally generous even on top of the law itself saying that killing a person isn't necessarily excessive. Whipping, mutilation, and the like are seen as basically normal and an enslaver who refuses to whip slaves could be seen as lax to the point that it might endanger the community. When a slave revolt, conspiracy, or rumors of the same go around, soft touch masters are right up there with abolitionists in causing it.

As a practical matter, enslavers had unlimited power over the lives of the enslaved and interfering with that was seen as dangerously close to abolitionism. It's only during Reconstruction that the power of the masters is transferred to state governments on a large scale.

Sources

Inhuman Bondage by David Brion Davis

Reconstruction by Eric Foner

River of Dark Dreams and Soul by Soul by Walter Johnson