Power resistor derating
The graph you refer to is the ambient temperature versus derating curve. This doesn't apply to the power dissipated by the resistor BUT it does assume that the ambient temperature arounf the resistor is maintained. For instance if the resistor were in a sealed bix the ambient temperature is inside the box and, if you ran the resistor at 200 degC then pretty soon the local ambient temperature around the resistor would be 200 degC and all bets are off.

You have to provide thermal managemtn for resistors getting this warm and that either means an open construnction enclosure such that air freely moves in and out, a fan or some other form of cooling system.

The problem you have is interpreting the thermal resistance figue of 88 degC per watt - this figure may assume that the resistor is soldered down to a "standard" PCB with some significant copper tracks OR it might apply to the resistor before the leads are cropped. The data sheet doesn't say (I didn't see this bit) so contact the supplier; Welwyn are a reputable firm and their is likely to be further info available from their web site.


  If the resistor will go over 200°C, is it still ok?


What does the ambient graph tells you - at 200 deg C you can dissipate 54% of the power - that's 1.62 watts and that would take the resistor up to 200 degC + 88x1.62 degC = 343 degC. Here lies two anomalies: -


That graph tells you that you cannot dissipate any power in the resistor at an ambient of 315 degC
The BS CE CC 40-201-002 requirements tell you that you can dissipate 2.5 watts at 70 degC. Given that TR is 88 degC per watt, it implies a resistor temperature of 290 degC.


You ought to clarify these with the supplier.