An interesting question! Firstly, do you mind if I correct a language error you've made? The word 'deaf' is an adjective - not a noun - so you can't refer to people as 'deafs' or say that someone is 'a deaf'. You can say 'I'm deaf' , or refer to 'deaf people'. As to your question, most English-speaking deaf people understand with a combination of 'lip-reading' and sign language. Of course, this varies from one person to another, and would depend on how profoundly deaf they are, and whether they were deaf from birth or lost their hearing at a later point. And regarding the question of the relationship between spelling and pronunciation, I don't think that this is a problem. As far as I know, British sign language is based on gestures which represent meanings and concepts , rather than the written word. A signer would only 'spell out' a word (usually on fingers) if it's an unknown word or a name of something or someone. American sign language is different from British sign language, but I believe the principle is the same. There are some people fluent in sign language who use the italki site, so they may be able to give you more precise information.

ASL Emerged from the Old French Sign Language and British from BANZSL ^^

No they can't, the American Sign language is from the family of French is more easy the American "speak" with a french or understand a little because is same base or become from the same than the British, the British sign language is from family: BANZSL

Your answer has really satisfied my question. Maybe their lip reading and speaking skill are due to various levels of deafness as su.ki said me under here.

I have heard that it is more difficult to lipread English than Italian or Spanish because so many sounds are not visible on the lips. In my experience working with deaf students, their lipreading and speaking ability varies widely.