Hi AS1975,
Thank you for the suggestions! The reason for limiting my study within the UK is rather practical and methodological. I use qualitative methodology, and specifically Interpretative Phenomenological Analysis, which means that I have to conduct face to face semi-structured interviews with participants, where the questions are partly formed based on the participant's responses. The online survey engines are great for quantitative studies, where you have many participants and are interested in statistical correlations. I chose qualitative methodology because I'm more interested in gaining an in depth understanding of individuals' subjective experience of what's like to be dealing with PTSD as a couple. So because the methodology I chose requires face to face interviews, I can only interview people in the UK where I'm based.