The quasar PDS 456 (at redshift ~0.184) has a prototype ultra-fast outflow (UFO) measured in X-rays. This outflow is highly ionized with relativistic speeds, large total column densities log N_H(cm^-2) > 23, and large kinetic energies that could be important for feedback to the host galaxy. A UV spectrum of PDS 456 obtained with the Hubble Space Telescope in 2000 contains one well-measured broad absorption line (BAL) at ~1346A (observed) that might be Ly-alpha at v ~ 0.06c or NV 1240 at v ~ 0.08c. However, we use photoionisation models and comparisons to other outflow quasars to show that these BAL identifications are problematic because other lines that should accompany them are not detected. We argue that the UV BAL is probably CIV 1549 at v ~ 0.30c. This would be the fastest UV outflow ever reported, but its speed is similar to the X-ray outflow and its appearance overall is similar to relativistic UV BALs observed in other quasars. The CIV BAL identification is also supported indirectly by the tentative detection of another broad CIV line at v ~ 0.19c. The high speeds suggest that the UV outflow originates with the X-ray UFO crudely 20 to 30 r_g from the central black hole. We speculate that the CIV BAL might form in dense clumps embedded in the X-ray UFO, requiring density enhancements of only >0.4 dex compared clumpy structures already inferred for the soft X-ray absorber in PDS 456. The CIV BAL might therefore be the first detection of low-ionisation clumps proposed previously to boost the opacities in UFOs for radiative driving.
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