CAMHS and CMHT
CAMHS stands for Children, Adolescent Mental Health Services and CMHT stands for Community Mental Health Teams.
If you have been referred to a local mental health service (CAMHS or CMHT) for an initial assessment by your GP it is likely you will receive an appointment for this within the a few weeks to a month of seeing your GP. This should not be a test to discover if you are 'really' trans or not - it is often more of a screening process to ensure you do not have any other mental illness that is contributing towards your desire to transition or to explore your gender related feelings.
The health professional will probably ask you some questions related to your family and life history. Try to be confident and answer questions honestly. You may already have been part of these services due to other mental health conditions that have or have had other needs for these services. Your gender identity feelings may come out as a result of engaging with these services.
In order to explore further your gender related feelings, or to continue exploring your transition, you may wish to see a health professional who has more expertise. At this point you will be referred on to Gender Identity Development Servicce (GIDS) or a Gender Identity Clinic (GIC). You do not have to be sure at this point about any part of your transition.