Rice[2]: £0.45/3650kcal[3]
Pasta[4]: £0.46/3710kcal[5]
Normalized to rice:
Potatoes: 3.79 rice_cost_units/kcal
Rice: 1.00 rice_cost_units/kcal
Pasta: 1.01 rice_cost_units/kcal
Tesco does not list calories for potatoes, and for the dry foods only provides calories for the cooked result, so I used values from the USDA nutrient database. The value for potatoes is optimistic, because the cheap potatoes are "imperfect", which suggests there will be significant loss from trimming inedible parts. Assuming efficient technique (pre-soaking dry foods, chopping potatoes small, reducing heat input to minimum once at temperature, covering cooking pots, using insulated pots if possible), cooking cost will probably be similar for all three, although pasta can be eaten "al dente" (undercooked), which might let it beat rice for total cost.
[0] https://www.tesco.com/groceries/en-GB/products/292332517
[1] https://fdc.nal.usda.gov/fdc-app.html#/food-details/170026/n...
[2] https://www.tesco.com/groceries/en-GB/products/300843799
[3] https://fdc.nal.usda.gov/fdc-app.html#/food-details/169756/n...
[4] https://www.tesco.com/groceries/en-GB/products/297844134
[5] https://fdc.nal.usda.gov/fdc-app.html#/food-details/168927/n...