British Israelism is the belief that the British are the 10 lost tribes of Israel. The doctrine has various expressions, some are antisemitic and others are not. I would be wary about any church that is not open about its beliefs, most have a statement of faith and list any organizations they are affliiated with. If the group is bigoted, they will probably justify their hatred of Blacks with the son's of Ham doctrine. There is a book entitled "One Blood" by Ken Ham of Answers in Genesis, [www.answersingenesis.org
] that refutes this belief both scientifically and biblically and explains that there is only one race, the human race. There is another doctrine called the serpent seed doctrine, which states that Jews are the descendents of Eve and the serpent, which you will probably find in this church. I can go through my materials and find some helpful information for you and I am sure the moderator has links to materials that may help you understand and explain these beliefs to your friends. I would not argue with them, just present them with some materials and ask them what they think.