Spending varies from person to person & you’ll receive a spending guide in your welcome pack, but 15-20 GBP per day is plenty on average. You can withdraw from ATMs at various points throughout the tour. This website gives accurate prices of all the basics, including food and alcohol.
Tipping is not compulsory, but is very much so expected in India. There is a culture of tipping around 10% of the bill for hotels and restaurants. We usually find it easier if your group leader says how much the tip should be and everyone can pitch in together to form a kitty which will cover you for any hotels or restaurants we visit on tour together.