The transition to sustainable aviation introduces new challenges in heat exchanger (HX) design, which is critical for managing thermal loads in hydrogen fuel cell and battery-powered propulsion systems. Unlike conventional propulsion, these systems generate significant low-grade heat, requiring advanced thermal management solutions. Moreover, as HXs become integrated into the aerostructures, they must withstand aerodynamically induced structural loads in addition to thermal stresses and internal fluid pressure, making their analysis a complex multidisciplinary problem. With the rise of additive manufacturing, the ability to fabricate complex, spatially varying geometries has enabled the use of lattice structures and repeated unit cells in HX design. This advancement facilitates multiscale analyses, where microscale material properties—including stiffness, thermal expansion coefficients, and permeability—are computed and incorporated into macroscale simulations through homogenisation. This study presents a multiscale and multiphysics computational framework for cross-flow HXs, capturing thermofluid and structural interactions at different scales. The microscale unit cell model is used to derive effective material properties and flow characteristics, which are then applied to macroscale simulations. Different architectures and configurations of the unit cell are explored to determine the limits of the homogenisation process. The resulting homogenised macroscale model is benchmarked against a full-scale HX model to ensure accurate predictions of global thermal and mechanical behavior. By integrating thermofluid, structural, and aerodynamic considerations, this framework provides an efficient and scalable solution for aerostructurally integrated HX designs in next generation sustainable aircraft. Future work will focus into aircraft mission profile-based studies, thereby optimising the HX design across the entire flight envelope.