Welcome to the Wefugees community and thank you for your question.
Once you are in the transit area of the airport you should get in touch with a Border Force (e.g. border police) and tell them that you want to claim asylum in the UK. Your application will then be registered and you will be screened (that's a meeting with an immigration officer). You should ask for an interpreter if you need one.
For further information about the asylum procedure in the UK, please check out this website provided by the British government: