This is always a delicate balancing act.
In theory, I would have more confidence that UK customs or police would not use these legislative powers inappropriately because the protections of the Westminster system of government that originated in England tend to be better than those of some other legal systems. Arguably, countries such as mainland China lack any such protections for individuals - or if they have these, the protections are ignored in practice.
However, there may be individual police or customs officials (in the UK) who overstep the mark on very flimsy grounds. This is where proper oversight comes into play, though I acknowledge that if one is trying to enter a country, border officials have what must seem to travellers to be enormous powers.
