Registering a company in India is a worse nightmare you can have so forget about it unless you really need it. You can easily work as freelancer and work with clients all over the world without messing around with the painful Indian bureaucracy and the corrupt system.
Invoice and receiving money is easy part. You can send invoice in £ and receive money in any bank account anywhere in the world as long as your bank allows to receive money from the UK. You can have contract between UK client and you as an individual.
Regarding your question for 2 contractors. You can have just one contract with the UK company and then deal with other contractors among yourselves. Don't give complex situation to the client.
Having a Pvt Ltd company is always tax efficient anywhere in the world however running it is messy in countries like India hence you would be avoiding it unless it becomes necessary. You should be thinking of setting up company if your business has grown enough and you need to get employees on board. If you are raking in, let's say, more than Rs 1 crore per annum then you should better consider setting up a ltd company provided you have future.