If we do not have any young people from the LGB community why do we have to do this ?

Tackling prejudice and stereotypes is the school’s job regardless as to whether members of the group are in the school.
The Public Duty of the Equality Act also requires the school to do the work. It is likely however, that though the young people are not identifying as members of the LGB community, there will be statistically such members who are either not out to you or themselves as yet.
There will be students who will have members of the LGB community in their family or social network. It is vital therefore that the school usualises the existence of LGBT people.