A great deal of thought goes into preliminary design - interactive as various fixed and consumable stores are positioned and re-positioned. The most important is the fuel and the fuel fraction as the designers/performance guys play with tankage - usually the most important consumable and most potential impact on static margins and cg travel for stability.
I would assume that the Hawker designers elected to go with their decision after exploring many options to improve hazard management for the amount of fuel required to meet the spec. Very few designers of Pursuit aircraft elected to put fuel in wings as wingtankage tended to force more weight into wing, make difficult decisions fat wing/thing wing for drag considerations, increase structural related weight, etc