Determining wattage of a variable resistor in series with a known resistor
If the 'variable resistor' is a transistor, always the same size regardless of setting, then you do need to compute the maximum power as you suggest.

If the variable resistor is a rheostat or potentiometer with a contact sliding along a track, then at different slider positions, you have different amounts of track to heat, and so different total power dissipations available. 

That is why such an element is rated for the maximum current the track can handle. 

For instance, in the datasheet of the RS part 8924913 you linked to, it says, in the characteristics section (my highlighting) ...

In order to calculate the max current, use the rated max resistance and the rated max power. For instance, a 10 ohm  100 watt resistor would take sqrt(100/10) = 3.2 amps. It will now take this at any setting.

Unfortunately, the data sheet above for this part is wrong. If you don't know the physics behind, or can't mathematically invert, \$P=I^2R\$, to get current as sqrt(P/R), then you will read the data sheet as current = sqrt(P)/R, which it does strictly say, as the surd does not cover the whole fraction. This will give the wrong answer. Slap on the wrist to data sheet writer. Cautionary note to all data sheet users, sometimes equations are not type-set correctly. When in doubt, apply physics.