despite their language clearly contravening the Race Relations Act
The Race Relations Act? Nah. There's an arguable breach of the Public Order Act 1986, but even then the threshold is much higher. Just using bad words isn't an offence. You'd need either to find individuals who will attest to having been harassed, alarmed or distressed (which are higher thresholds than "offended") or you'd need to show intent to stir up racial or religious hatred, which is again a much higher threshold (because of the "intent" part).
People appear incredibly quick to throw around claims of things being illegal without there being a hint of a prosecution, and use of a racial epithet isn't close to triggering prosecutions.
Amazon are selling the pictured tee-shirt ("Pikey: just nick it") and there's no sign that either they are being prosecuted for distribution or that people who buy it are being prosecuted for wearing them. In fact, I'd say that Nike have more chance of a civil action for trademark issues than anything else. Rather like the endless "call 111!" threads, people seem to have unrealistic views of what is actually illegal. I'd rather people didn't have stand up routines that started "So, a kike, a nigger, a coon and a pikey go into a bar..." and I think that people who engage in or enjoy such "humour" should be shunned by polite society. But it isn't, and shouldn't be, illegal. There isn't a law against being obnoxious and ignorant.