"您贵姓？" is not a yes/no question; therefore, there is no need to add 吗. “他们是英国人吗？” means "Are they British?" Instead, "他们是英国人？"(with a rising tone at the end) is equivalent to "They are British?", expressing a doubt.

I'd like to share an interesting thing about 吗 with you to help you use it. If you want to say "blablabla 是 blablabla (吗)?" but you do not sure whether it is correct to add 吗 or not. DO NOT ADD AND USE THE TONE.It is more native in many situation espeacially when you are talking with your friends:)

Well, it may be a little complexed to explain. If you translate "您贵姓" into English it is "What's your family name?"; "他们是英国人（吗）？" is "Are they British people?" (here you can add "吗" or just ignore it). So, can you see the difference? In general questions you can choose to end them with "吗" or not (but always rise your tune), while in special questions, e.g. 谁 (who)/ 什么(what)/ 何时(when)/ 何地(where), grammatically you don't add "吗". Instead, you can end your questions with "呢" or "啊" to make them sound more soft and euphemistic. Hope this answer will help you.

For those sort of questions like "Is it cold outside?" or "Is he British?"....that you can answer just with "yes" or "no", plz add the "吗“. But for the special questions ( i'm not sure if this concept exists in English) like "Which flower do you like?" "Why do you love me? she is way better than me.".....just drop the "吗“