How much would petrol and insurance cost in the UK?

Basically my friend and I are spending about 3 months in the UK and are thinking about buying a car so we can get into the city. We intend to share it and both have licenses but never owned cars before. We could split a car costing about 400 pounds but I was wondering whether it would be worth it? We intend to live just outside of London and work in the city. If we carpool into town eveyrday about how much would petrol and insurance cost us? My friend might keep the car when he goes off to uni.