OT departments will normally accept referrals from a range of health and social care professionals: http://www.nhs.uk/Conditions/occupational-therapy/Pages/accessing-occupational-therapy.aspx
Often the GP will be centrally positioned in supporting a person's health care needs, co-ordinating referrals, but other professionals can make referrals if they can see there is a clinical need.
As @stayce has already pointed out OT can be perceived as an expensive service (despite the fact that research shows that OT interventions actually save the NHS money overall). Also there is a national shortage of OT's. It appears that because of the perceived cost implications and sometimes limited availability there can be gatekeeping that goes on, in effect, to ration the service.
If you are still struggling to get a referral, despite asking the school nurse etc to support you, I would suggest that you make a double appointment to see the GP so that you have time to talk. Explain the situation to the GP, explain how you feel your child is being denied a service that you feel is essential, due to a reluctance of professionals to take responsibility for a referral. Ask the GP to make a referral, if they refuse, ask them to explain precisely why. Is it because they don't feel your child meets their referral threshold? or is it because they think another individual/department should be making the referral? If it is the latter, ask the GP to write to that person/department saying that in his/her opinion your child would benefit from them making a referral to OT.
Unfortunately, chances are that even when you child eventually gets a referral there may be a wait of several months before an appointment is received.
Are there any pressing issues that you are hoping the OT might be able to address that in the meantime you could get support for from another source?
It would be interesting to hear if you manage to get this problem resolved
Jean Merrilees BSc MRCOT
You can read more of my posts at: https://community.scope.org.uk/categories/ask-an-occupational-therapist