No, paying cash does not count as the fidyah for not fasting. Actual food must be given because Allah says: “And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day)”
Crisis Aid gives out actual food. We can feed the poor and needy on your behalf and the cost for fidyah is £3 per day. Please calculate £3 per day by the number of fasts you need to make up for.