Hello gringaingreen,
There are many Churches with services in English. There is Saint Michael's and All Angels Anglican Church in Martinez, Rev. Brian Williams, (Sarmiento 328). Also, there is St. Andrew's Presbyterian Scots Kirk in town (Belgrano 579), tel: 4331-0308, Rev. Kenneth Murray. Also you could attend Services in English at the United Commnity Church in Acassuso. This is the American Church. There is another American Church in Martínez, on Sarmiento (just a few blocks away from St. Michael's, near Av. Libertador, but I don't know anyone there.
In Belgrano, you could go to St. Saviour's Anglican Church (Crámer 1816) Tel: 4783-0272.
I think you will find more thorough information at
http://www.buenosairesherald.com . Go to the "Community Life" section.
I hope this helps.
Cheers
Ernie