Buses are a popular transportation medium in London. You can use an Oyster Card or a Day Travel Card to travel with them.
Depending on your hotel location you can use a variety of buses to get to the church. We would recommend going on www.tfl.gov.uk and use the Journey Planner option on the right side of the webpage. Simply type in your starting point and your destination. Click "leave now" and see the results. You can now "edit the search options" and only select "buses". Now you can view the best bus options.
If you are planning on spending a few days in London, we would recommend purchasing an Oyster Card. These are available at any tube station and basically work as a 'pay as you go' system. Each time you use the card, money will be deducted and you can top up as little or as often as you like. For more information on the Oyster Card visit TFL's website.