SIM cards are easily available to tourists in the UK, with low cost calling to India. However, you may want to buy them in town rather than at the airport.
The good news is that prepaid SIM cards are easily available for tourists, and there is no document of identity and address needed to buy them.
They are available at the airport but they are cheaper to buy from the town rather than from vending machines at the airport. In town they are available at general stores, news agents, grocery stores and even at petrol stations. Look for this image for a SIM card.
You can also order a free Pay As You Go SIM card through the internet before you arrive in the UK. Unfortunately you’ll need to provide a UK-based address for the SIM card to be sent to as SIM card providers do not ship abroad