TSR.2's downturned wingtips were a simple alternative to normal high-wing anhedral (as seen on the Mirage F1, for instance). Basically, they did the calcs and found that the weight saving from making the wing box flat, straight and simple was worth the slight complication of the downturned tips. It is, in fact, exactly the same logic (but upside down) that lead to the F-4 Phantom's upwards kinked outer wing panels.
Either low or high mounted wings suffer undesirable effects in sideslip, due to air "piling" up against the fuselage, hence most high-wing aircraft have anhedral, most low-wing aircraft have dihedral, and most mid-wing aircraft have neither, since the effects above and below the wing cancel each other out. There are exceptions, of course, depending on the detailed aerodynamics of the aircraft in question.
There are plenty of other design reasons influencing the choice of high, mid or low wing. High, or at least shoulder-mounted, wings are common on high-agility types primarily because at high AoAs they leave a low tailplane in nice, clean airflow, rather than in the turbulent wing wake.
