How can u get married in a church in London?

My fianc� and I would like to get married in a church in London, but neither of us know what the next steps should be. As well as that we are very time limited and need to get married by the mid august. Could anyone give us an advice on the approximate costs - for the ceremony we plan to have 70 people at most and reception about 150. And basically what should we look at, thanx