The Church of England uses a St Georges Cross with the arms of the relevant deaconess in the canton, (in this example the deaconess of Canterbury).
The Church of Australia feature a St Georges Cross with the traditional headdress of a Bishop. The blue field reflects the colour of the national flag, with a commonwealth star which represents all the states and territories in each quarter.
The Church of Ireland simply uses the Cross of St Patrick although it the UK flag has been observed being flown from Church of Ireland buildings in Northern Ireland.
The Church of Scotland uses the Cross of St Andrew defaced with its Burning Bush insignia from the Book of Exodus
The Presbyterian Church in America uses a flame cross logo rather than the bush used in Europe on a white field
Although the Churches in America also seems to use unofficial flags like this one with the cross logo on the cross of St Andrew.
According to Flags of the World the official flag of the Roman Catholic Church is an elongated bannerette type flag divided into a gold and white field.
However I think most people are more familiar with the flag of the Vatican City, and associate more with it.
Greek Orthodox Church
There have proposals for an interdenominational Christian flag such as the one below which appeared in the early 20th Century USA.
This is a popular flag with churches of various different denominations throughout America, the symbolism of the cross in the canton is self explanatory, the colours represent the blood of Jesus (red) faithfulness (blue) and purity white. It is sometimes defaced with the symbol of church or denomination in the fly. Although it is used in the USA, Latin America and to a lesser extent in Africa it is little used in Europe and Asia (possibly because of its similarity to the American flag).
My proposal for a Christian flag is simply a white field with a gold cross:
Or a fish which was used in the early days of the Church when the Christians were persecuted by the Romans, because there are unfortunately parts of the world were people of various faiths are prosecuted, this could be used where people for whatever reason might want to identify themselves to other Christians but at the same time try to disguise the fact from others:
or alternatively a trexelon or trillium to represent the three faces of God (Father, Son and Holy Spirit).