Getting a SIM card in the UK is much easier than being a resident in India @reelssaver . In the UK we don't have to produce all the documentation that is needed in India.
The problem is you will probably need to keep to PAYG for a while, to get a pay monthly contract, Vodafone will complete an identity and credit check and you are expected to have had a UK address for a while and be on the UK electoral roll at the address, plus you will need some UK credit history to base the check. Vodafone have information on credit checks here Credit Checks
As far as PAYG is concerned you can order a free SIM here: Free SIM .
Take a look at Voxi here Voxi still Vodafone giving you unlimited Social Media and it'd almost the same as having a contract without the need for any credit check, you register your UK credit or debit card and Voxi take the monthly payment.
When you arrive in the UK you can simply go into a store without all the documents you have to produce in India though it wouldn't do any harm to take your passport and purchase a Vodafone PAYG or Voxi SIM. There is a list of stores here: Vodafone Store Locator
To add : See here Calling Abroad from the UK well worth adding an extra to save money Vodafone extras
