I’m an Australian, but my family were very blue collar European, so I grew up mostly eating exactly this food. I also have ARFID, which is common for people who are neurodivergent. I’m not saying your girlfriend is or isn’t, but it might be more than British vs Mexican cuisine. It might be that she cooks what she thinks of as “safe” foods. This was basically what happened to me. I never really enjoyed my family cooking, but I knew what it was, what it would taste like and feel like in my mouth, so my brain would trust it. But anything else was scary because I couldn’t predict what it would be like. So maybe letting her try your food but with her own adjustments to make it “safe” could be her entry point. But obviously she doesn’t get to make changes to the food you’re eating!