Plant-based diets are seen as a food-based strategy to address both the impact of dietary patterns on the environment, to reduce climate change impact, and also to reduce rates of diet-related disease. This study investigated self-reported consumer purchasing behaviour of plant-based alternative foods (PBAF) and wholefood plant protein foods (legumes) with a cross-sectional online survey. We identified the sociodemographic factors associated with purchasing behaviour and examined knowledge about protein and plant-based diets. We recruited and obtained consent from n = 1177 adults aged >18 from England and Scotland (mean age (± standard deviation (SD)) 44 (16.4) years), across different areas of social deprivation, based on postcode. Descriptive statistics were conducted, and sociodemographic factors were examined by computing covariate-adjusted models with binary logistic regression analysis. A total of 47.4% (n = 561) consumers purchased PBAF and 88.2% (n = 1038) wholefood plant-proteins. The most frequently purchased PBAF were plant-based burgers, sausages, and mince/meatballs. Individuals from low deprivation areas were significantly more likely than individuals from high deprivation areas to purchase wholefood plant-proteins (odds ratio (OR) 3.46, p = 0.001). People from low deprivation areas were also more likely to recognise lentils as good source of protein (OR 1.94, p = 0.003) and more likely to recognise plant-based diets as healthy (OR 1.79, p = 0.004) than those from high deprived areas. These results support current trends of increasing popularity of PBAF, which is positive for the environment, but also highlights these products as being ultra-processed, which may negatively impact on health. The study also re-enforces the link between deprivation, reduced purchasing of wholefood plant-proteins and knowledge of plant-based protein and diets. Further research is needed to examine healthfulness of PBAF and how sociodemographic factors, especially deprivation, affect both food choice and consumption of wholefood plant-proteins.