A current trend in the evolution of mobile communication networks consists in integrating Non-Terrestrial Networks (NTN) with the Terrestrial ones. One option to implement the NTN part of this hybrid architecture using Unmanned Aerial Vehicles (UAV) that relay the uplink radio signals through optical wireless backhaul links. A good choice for the radio uplink waveform is conventional SC-FDMA, which mitigates the PAPR and enables a longer battery lifetime at the transmitter side. For the optical backhaul link, which is based on low-cost Visible Light Communication (VLC) technology, a non-orthogonal implementation of SC-FDMA is proposed. By doing so, it is possible to improve the end-to-end throughput by reducing the communication bandwidth (to make it fit the LED frequency response), mitigate the effect of light reflections, and increase the energy efficiency in the backhaul link. Since VLC relies on non-coherent IM/DD, the non-orthogonal SC-FDMA waveform must rotate the phase of the IDFT subcarriers, in order to obtain real-valued signal samples at the output. Two strategies for relaying the data in the UAV node are evaluated, namely: Detect-and-Forward and Decode-and-Forward. The first one recovers the modulation part (i.e. partial regeneration), whereas the second one regenerates the transmitted message up to the bit level (i.e., total regeneration). This paper studies the combination of relaying strategy and NB-IoT Modulation and Coding Scheme (MCS) that maximizes the end-to-end throughput at different UAV altitudes.