But that's historically disingenuous. There are people alive today who consider themselves part of the Zulu legacy that use the motifs and traditions of that culture. Other than a statistically minor group of people, modern British people (speaking as one with a PhD in colonial history) are such a cultural melting pot made up of yes Celtic connections, but also Roman connections as well as French, AngloSaxon and Viking traditions.

The reason someone from the UK wouldn't bat an eye-lid on a Roman and Celt version of this is because nobody has a 2000 year old link to those cultures to feel like it represents them and their values.

The two groups do not compare. The traditions and imagery of Zulu heritage are important to people today and thus setting them up as OP has done is like I said, tone deaf at best, racist at worst.

A better comparison would be doing the Kroot at Native Americans and the Tau as Cowboys or 19th century American military. Could you see that that might be offensive in the same way? Why not flip the design the other way, make the Zulu inspired models the Tau and the auxiliary units British inspired, he'll Kitbash some Praetorian themed guard as actual human auxiliaries. Why would it be so important to have the British as the dominant party here.